Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from Postgrad Med J (1991) 67, 334 - 346 i) The Fellowship of Postgraduate Medicine, 1991

Reviews in Medicine Louis Kreel

Department ofDiagnostic , Prince of Wales Hospital, Shatin, N. T., Hong Kong

Introduction Physicists have tapped the electromagnetic spec- latitude with lower radiation and minimal repeat trum to great effect. Each energy wave-form that examinations. Dark rooms for film processing will penetrates tissues has a corresponding imaging disappear. too will be fully automated system, starting with Roentgen's radiograph of his and digitized, and all radiographs will show the full wife's hand in 1895. Since then gamma rays, range of radiodensities from lung, through soft X-rays, protons, ultrasound and radiofrequency tissues to bone with one exposure. This will entail coupled to a magnetic field have been grafted onto new equipment, film, cassettes and imaging systems computers to produce anatomical images varying at considerable expense. in type and detail. More recently these modalities Furthermore, each passing year sees the intro- have produced sophisticated physiological data of duction of new scanners with shorter scanning considerable interest. times and higher spatial resolution, producing Thus the words 'digital' and greater detail and 'computer' have more information. Millisecond copyright. invaded radiology departments, not only with computed (CT), cardiac cine, scanners, of which they are integral components, magnetic resonance imaging (MRI) down to but also in conventional fluoroscopy and radio- breath-holding time, and the technology of CT graphy. The change is as revolutionary as that of applied to (NM), yielding single steam engine to internal combustion engine or photon emission CT and proton emission tomog- automobile to aeroplane. The costs too are com- raphy, are now available. parable for the equipment, although in the long run With this equipment and the development ofnew

the cost ofpatient care will remain unchanged or be radionuclides as well as MRI spectroscopy, which http://pmj.bmj.com/ even less ifthe rapidity and accuracy ofdiagnosis is is at present being evaluated in clinical practice, taken into consideration. windows to cerebral activity in health and disease So much for analogies. What, then, in broad have been opened. The research emphasis in imag- outline is new? PACS, the picture archiving and ing is thus changing from morphology to function, communications systems, if fully operational, will particularly in the heart and brain. create hospitals without radiographic film. Images However, the great and elusive goal of tissue will be transmitted instantaneously within hos- characterization has not been fulfilled with either pitals, between hospitals, across the country and CT or MRI. The search continues with magnetic on September 28, 2021 by guest. Protected between continents, with greater contrast and resonance spectroscopy (MRS) in the hope of resolution by tele-video communication. Archiving distinguishing granulomas and tumours, benign and recall will also be nearly instantaneous. In from malignant, or even to go further by assessing addition, the clinical details, biochemistry and the degree of malignancy. The ultimate aim of available pathology will appear on the screen. diagnostic imaging then is to produce complete Radiology departments will become diagnostic morphological and functional information, departments of medical imaging. thereby equalling the achievements of histology. In the interim, conventional is being MRS and NM individually or in combination are automated, with concentrating on also being applied to metabolic diseases. the patient's well-being, on reassuring the patient The magic and art of imaging resides with and on careful positioning. The radiographs will be sonography, where the hand-held probe sees processed in daylight, having greater definition and directly into the body, now extending its scope even further with colour Doppler, endoscopic sono- graphy and peroperative applications. As the centenary of Roentgen's discovery Correspondence: L. Kreel, M.D., F.R.C.P., F.R.C.R. approaches we can perceive how X-rays are slowly Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from MEDICAL IMAGING 335

but surely being ousted from their pre-eminent Video consoles are an essential component ofall position in medical imaging, giving way to non- scanners. The images are seen in real time during ionizing electromagnetic energies. In fact it is sonography, almost in real time with the newest CT extremely doubtful if a modern ethical committee and MRI equipment, then shown as static images would allow the use ofX-rays in clinical practice if for interrogation or as a dynamic cine loop study. presented as a new discovery. On the basis of Nuclear medicine and Doppler sonography are animal experiments the effects of X-rays would often shown in colour. Yet very few departments probably be considered unacceptable. In the his- have a central viewing area where the various tory of medical imaging the 20th century may well images can be integrated. be designated as the century of iatrogenic radia- At present, then, these video consoles are part of tion, medical imaging having replaced radiology. the equipment used during the examination or only within the confines of the particular diagnostic suite and are not available for interdisciplinary The radiology department consultations when the information from 2 or more examinations needs to be correlated, especially The production, filing and delivery of radiographs and CT or the various sequences of to the wards, out-patient department and theatre MRI. The correlation of sonograms with other are tedious and labour-intensive tasks. Subse- scanning procedures and isotope scans with plain quently their retrieval for re-filing is often difficult films are other examples. The advantages for and fraught with recriminations: the vital and consultations and teaching are self evident. interesting film is always the one that is missing or A system that will permit the simultaneous lost. Storage of film requires a large space, partic- viewing and integration of the various images ularly as radiographs must be kept for at least 7 necessitates their on-line transmission to a central years. Many departments and hospitals would like archive whence they can be viewed at one or more to have follow-up studies for even longer periods, areas such as a consultation studio or the surgical especially orthopaedic surgeons and chest operating suite. In the first instance the images

physicians. Frequently operations and consulta- must be available to radiologists for their immed- copyright. tions cannot proceed without the relevant films. iate assessment to conclude the examination and This holds true also for interdisciplinary case for reporting. Simultaneously the images are stored presentations, particularly for tertiary referrals, in the central archive where the various modalities Furthermore, transmitting radiographic inform- are integrated for future display. ation verbally or in writing is often ambiguous. To be effective the archive must be able to hold Picture archiving and retrieval systems (PACS)' the vast amount ofinformation and to reformat the can eliminate these difficulties, but require the data image. Either more powerful computers are needed

in digital form. or there must be compression of the images but http://pmj.bmj.com/ Most medical images are or can be digitized, with no loss of detail. Such compression systems including not only NM, CT, MRI and obviously have now been produced with ratios of 5-10:1. digital subtraction angiography (DSA) but also, in The transmission of medical images from one the not too distant future, plain film radiography. location to another is called teleradiology' and Digital images have many advantages for viewing, wide area networks are already in existence. A laser storage, retrieval and transmission. film digitizer with an optical disc and a laser printer The image on a video monitor can be modified in are essential for high-quality images. A variety of a number of different ways and interrogated. Part technologies are used for the communication net- on September 28, 2021 by guest. Protected or all of the image can be enlarged or diminished, work, including co-axial cables, fibre-optic links, measured, annotated, reversed, and windowed to switched circuits and satellite transmission pro- show the different tissue densities of bone, soft viding immediate images to the accident and tissue or lung, and these densities can be measured. emergency department and intensive care unit. Sectional images can be reconstructed into other Outlying hospitals or clinics can communicate with planes or into D-3 and optimized for spatial and their large central counterparts or academic insti- contrast resolution. Hard copy of these images is tution before admission is arranged. frequently a poor substitute for viewing the images In the United Kingdom and many other coun- on a monitor, resulting in a loss of important tries these major developments will concern only a information. The small subdural haematoma is an small number of institutions. Many more will be example. Unless the CT sections are shown at a affected by the prospect ofthe greater resolution to wide window, it will be indistinguishable from be obtained with conventional radiographic equip- bone. There are a number of other techniques that ment.6 8 In one system the film itself has been also depend on the manipulation of digital data, changed and in the other radiographic film has such as cardiac and respiratory gating and forming been discarded in capturing the initial image. A cine loops for dynamic studies. radiographic image is produced by transmitted Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from 336 L. KREEL

X-rays falling on halide crystals in the emulsion of pleural effusions. The fluid acts as a window to the film after the fluorescent screens in the cassette underlying lung and mediastinal disease for con- have been activated. In the Kodak system the solidation, collapse and tumours.2526 In localized crystals are flat and polygonal, producing a much pleural disease small effusions can be distinguished larger surface area to receive the incoming photons from solid lesions, and ultrasound is also ideal for and hold the image. This film is combined with accurate percutaneous aspiration or biopsy. rare-earth phospher fluorescent screens to produce Transthoracic cardiac sonography27 31 is used in a green -sensitive system. The resulting radio- valvular disease, pericardial effusions, myocardial graph has better definition and increased dynamic motion studies and septal defects. The scope of range, allowing visualization of both the medias- sonocardiography has been increased with colour tinum and the lungs on a . In a Doppler, which reveals the direction ofblood flow. further development the advanced multiple beam Intracardiac shunts and valvular disease can be equalization radiography (AMBER) system uses a assessed more accurately. Transoesophageal sono- rectangular multi-X-ray beam to scan the thorax. graphy has proved itself in the diagnosis of aortic The wide range of radiodensities is equalized by a lesions, particularly aortic dissection,32 as a rapid, microprocessor. The system also requires a 3-phase accurate method that also gives a good view of the generator with an output up to 150 kVp and an 0.8 left atrium, left atrial appendage and left ventricle. second exposure. Intra-operative sonography allows accurate assess- With their FCR 7000 system Fuji have gone one ment of the aortic and mitral valves necessary for step farther and eliminated the film in capturing the decisions on whether to perform valvulotomy, initial image by using computed radiography and valvuloplasty or valve replacement. high-sensitivity plates read by a high-precision spot Transoesophageal sonography is at present the laser system. The image can then be transmitted to best method for staging oesophageal carcinoma,33 a video console or printed out on film as a particularly in assessing the depth of tumour radiograph that shows both soft tissues and bone invasion and the presence of lymphadenopathy, on one film, or two films can be processed simul- and can also show whether sclerotherapy or varices taneously. The films have greater definition and the has been effective. Further down, transgastric copyright. radiation dose is reduced to 10% of a conventional sonography is similarly effective for gastric tu- chest radiograph. mours34 and also in pancreatic tumours including The cost ofimmediate replacement ofthe present insulinoma. Recent reports indicate that intra- radiographic units by these newer systems would be operative sonography has a high accuracy in beyond the UK National Health Service in the locating a deeply seated endocrine tumour shown present financial blight but might be achieved as as a well-circumscribed hypoechoic area within the existing equipment wears out. However, the much .35 lower radiation dose is a powerful argument for an Sonography is indissolubly linked to the modern immediate change in paediatric examinations. practice of obstetrics and gynaecology.36 In obstet- http://pmj.bmj.com/ rics the fetus and placenta can be imaged almost from inception to parturition, and assessment can Sonography be made of fetal gestational age, growth and viability, multiplicity, fetal abnormalities and pla- Ultrasound examinations are fast and inexpensive cental localization, texture and transplacental hae- but are much more dependent on the operator than morrhage. Internal pathology such as pelvic masses other scanning methods. They are particularly can be diagnosed. The most remarkable images are on September 28, 2021 by guest. Protected useful for localizing mass lesions, whether cysts, produced by transvaginal sonography showing the abscesses, granulomas or tumours, benign or ovaries, adnexae and uterus in great detail, which is malignant, and for percutaneous aspiration, cytol- extremely useful in the diagnosis of ectopic preg- ogy and biopsy.9 14 Newer units have much better nancy.37-39 If pregnancy is confirmed by the sen- definition, providing more accurate diagnosis. sitive and specific radioimmunoassay for the beta In recent years sonography has to a large extent subunit of human chorionic gonadotrophin (P- replaced contrast examinations of the gall blad- hCG), intravaginal sonography with Doppler can der15 20 and genito-urinary tract21'23 and is in most exclude an ectopic pregnancy by locating a gesta- instances more helpful in the abdomen than plain tional sac within the uterus, and it can be distin- film radiography.24 Sonography has an important guished from a pseudogestational sac by its fre- advantage over radiography in that it is a non- quency shift. Other signs such as pelvic fluid and ionizing form of radiation. endometrial changes are not helpful. The actual While gas and bone cannot be penetrated by ectopic pregnancy in the adnexae can be visualized ultrasound, there are still many indications for its in about 40% of patients found to have an ectopic use in the thorax in addition to sonocardiography, pregnancy at surgery. and it is especially indicated prior to tapping of Transvaginal endosonography is also consid- Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from MEDICAL IMAGING 337 erably more accurate than transabdominal sono- cular pattern as in hepatomas.54 graphy for staging of cervical carcinoma and in Sonography is especially valuable in paediatrics assessing the ovulatory cycle for in vitro fertiliza- not only because there is no radiation hazard, but tion. At the time of oocyte transfer, transvaginal also because it is non-invasive, rapid and accurate sonography allows harvesting of oocytes by ac- and can be applied to all anatomical areas includ- curately localizing the ovarian follicle.' ing the scrotum for the diagnosis of testicular The advantages of transvaginal sonography are torsion." Children have the ideal combination for manifold. The uncomfortable fully distended blad- ultrasound because of their poorly developed fat der is no longer necessary, reducing waiting time planes and small size. Furthermore, the reassuring for patients. The high-frequency (6 MHz) pencil- parent can be close to the child. shaped probe has an effective focal zone of up to Sonography, therefore, if used wisely and well, 8 cm, producing excellent detail, earlier recognition becomes the most commonly used and most ver- of the fetal heartbeat and recognition of a non- satile scanning procedure, with a constantly in- viable pregnancy. Advanced masses are more creasing scope especially in neonatal practice and accurately assessed. in the intensive therapy unit. It is the only portable Transrectal sonography for anorectal4' and pro- scanning method that can be used in the operating static42,43 carcinoma is equally effective for tumour theatre to localize brain pathology,56 on restless ill localization and staging. The peripheral, central patients that cannot have CT or MRI, and on and transitional zones of the prostate can be premature babies on life-support systems.57 identified with the 5 and 7 MHz probes as hypo- The magic and art of imaging resides in the echoic areas. The zonal concept of McNeal' is ultrasound suite. extremely important as 70% of tumours occur in the peripheral zone and only 15% in the inner transitional zone. Under antibiotic cover transrec- Computed tomography tal biopsy is safe provided there is no evidence of active infection and no bleeding tendency. It is As the name implies, CT could not exist without

probably at present the best screening technique computers. Simply stated, a rotating X-ray tube copyright. with a specificity of about 95%. provides an X-ray beam that passes through the Transcutaneous sonography of the abdomen for body stimulating an array of detectors. The , pancreas, spleen, gall bladder and kidneys is absorbtion of the X-rays within the body or well established and needs no further emphasis, attenuation ofthe beam can be calculated from the apart from stressing its role in abscess localization difference between the entering and emerging and drainage, especially peri- and intrahepatic photons, and by Fourier transform the attenuation collections, and in percutaneous cytology and of each small volume or voxel or tissue can be

biopsy.45-4' However, there is a definite role for computed and localized within a section of the http://pmj.bmj.com/ sonography in and around joints, tendons and body.58 muscles. Joint effusions, capsular thickening, ten- The digitized information is then converted into donous tears, calcification, muscle tumours, an anatomical image, almost always in the axial haemorrhage and abscesses can be displayed. In the plane except in the skull, where coronal sections are shoulder impingement syndrome the fluid collec- used especially to display the orbits, pituitary and tions in the subdeltoid-subacromial bursal system paranasal sinuses. and the lateral pooling of fluid in the subdeltoid The image is displayed on a video console and part when the arm is raised are diagnostic.4849 can be manipulated or interrogated because the on September 28, 2021 by guest. Protected Colour Doppler has been mentioned for cardiac information is essentially digital. By 'windowing' lesions, and now with pencil or annular phased the image the grey scale can be changed to produce array probes (5-10 MHz) dynamically focused either greater or lesser contrast or spatial resolution with a symmetrical cylindrical beam blood vessel and can be set for tissues of varying radiodensity. imaging becomes even more accurate. Pulmonary The same data can then be used to show the full emboli from deep venous thrombosis are an ever- range of tissue attenuation from lung through fat, present hazard in hospital whether in post-surgical water and soft tissues to bone without rescanning or medical patients. Non-invasive techniques, if the patient. Many other manipulations can also be effective, must take precedence over contrast done, such as obtaining the actual attenuation , and Doppler sonography is proving values in Hounsfield units, measuring distances itself in this sphere.50-52 The aorta and its major and areas, using special programmes for bone branches, renal, mesenteric, coeliac axis, hepatic detail and for reconstruction into coronal and and iliacs, and the measurement of portal blood sagittal planes. These facilities with CT have been flow53 can be imaged and interrogated for blood- available for about 15 years since the first body flow data. Doppler is valuable in the differential machines were produced that could scan within diagnosis of tumours by demonstrating the vas- breath-holding time, i.e. about 18 seconds.59 Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from 338 L. KREEL

More recent refinements include scanners that Interstitial lung disease can now be categorized produce an image in 1-2 seconds, completing an by the axial distribution, by the appearance of the examination of the abdomen or thorax in 8-20 abnormal densities and by the effect on the secon- seconds with almost simultaneous or on-line dis- dary lobule.66 As a practical detail, perihilar or play of the images. Reconstruction into 3-D, central disease requires transbronchial biopsy, creating holistic images, is a still more recent mid-lung disease requires open lung biopsy, and development that offers the ability to view from peripheral disease can be diagnosed by per- any angle and show the various anatomical layers cutaneous lung biopsy. The distribution of bullous without superimposed tissue. emphysema is clearly visible. Quite recently CT equipment has been produced On the basis of chest radiography and thin with no moving parts by means of the principle of section CT, pulmonologists now describe abnor- the 'electron gun'. Focused electrons strike an malities as linear, reticular, nodular, reticulo- array of targets that send the photons through the nodular, ground glass, homogeneous or honeycomb. body to the detectors. With this system images can Their distribution is referred to upper, middle and be produced in milliseconds and a whole examina- lower zones as well as to peripheral (cortex), middle tion can be completed in seconds, ideal for contrast or hilar (together forming medulla). The more studies of the heart.' A further innovation is the accurate localization, the earlier detection67 and introduction of thin section CT of 1-2 mm in more exact descriptive terminology constitute a conjunction with scan times of 1-2 seconds. The major advance in chest radiology and pulmonary resulting lung images have exquisite anatomical diagnosis.68 detail.6" The ultrafast CT scanner (Imitron) can produce The most significant drawback of CT is that an image in 50 milliseconds and complete an contrast medium is needed to show the cardiovas- examination of the heart in under 1 second, taking cular system. While CT equipment is about half or 17 sections per second to produce cardiac cine in two-thirds of the price of MRI, it is some 5 -10 real time.69'70 Ventricular motion, cine-angio- times the price of sonographic equipment. graphy and ejection fractions7i can therefore be Maintenance costs have a similar differential. studied by CT with great accuracy, which is However, CT images are immediately recog- especially valuable in assessing cardiac shunts72 and copyright. nizable, as the organs are well demarcated by fat patency of coronary by-pass grafts. Very early planes, the bowel can be labelled with oral contrast coronary artery calcification can also be detected, medium, and the appearance of the skeleton is said to predict the presence of coronary artery similar to that seen on conventional radiographs. stenosis. Aortic dissection is well demonstrated by Both sonographic and MRI images are totally fast scanners that have a further advantage: much different from each other and unlike CT are not less contrast medium is needed and can be given as immediately recognizable as anatomical sections. a single bolus without the need for a multiple bolus CT has been accepted by all the medical special- technique or rapid drip infusion. http://pmj.bmj.com/ ties as an important diagnostic tool, most of all for The most important development in CT tech- neurologists and oncologists as it can be and has nology, however, is the production of a cheaper been applied to all parts of the body.62'63 The main versatile model, more compact, that no longer limitation, as with all imaging systems, is the requires a separate air-conditioned room for the inability fully to characterize tissue, particularly in computer. Computed tomography becomes a distinguishing benign and malignant lesions- a reality for small general hospitals. recurring problem in the treatment oftumours. The on September 28, 2021 by guest. Protected residual tissue after radiotherapy or chemotherapy is readily demonstrated by CT but fibrosis and viable tumour appear similar. Further manage- Magnetic resonance imaging ment relies on a 'wait and rescan' policy, surgical excision or percutaneous biopsy.'M Sonography is at the lower end ofthe cost spectrum Interstitial lung disease has an extensive in imaging and MRI at the uppermost. Yet in the differential diagnosis including sarcoidosis and United States there is ready access to this equip- other granulatomous diseases, allergy, collagen ment, particularly in the major academic institu- vascular disease, haemosiderosis, proteinosis and tions and in private clinics, resulting in an extensive malignant infiltration. With thin section CT of literature. The technology is ever expanding. 1-2 mm, using the bone algorithm and a scan time MR images are produced by the combination of of 1-2 seconds the secondary lung lobule can be a magnetic field and a radio-frequency pulse. As in visualized65 with a central dot representing the sonography, the pulse generator also acts as the centrilobular artery and bronchus. The outer receiver. Three types of magnet, permanent, resist- polyhedral margin ofconnective tissue contains the ive and cryomagnet,73 varying in strength from 0.1 veins and lymphatics. to 2 Tesla are available and vary in cost from £0.75 Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from MEDICAL IMAGING 339 million to £2.0 million. The radio-frequency coils The value of MRI was recognized immediately are either large body coils or small and localized to when demyelinating lesions were demonstrated fit specific anatomical areas. The computer soft- that were not seen with CT, especially in multiple ware creates the pulse sequences to form the sclerosis.82 Other advantages were also obvious. images.74 The regions abutting bone, the cerebral cortex, In brief, within the body, the protons of the base of brain, and the posterior fossa were shown water molecule act as magnets, normally in a quite clearly, whereas with CT these areas are random distribution. When in the MR magnet the poorly demonstrated, often by superimposed protons are aligned in the direction ofthe magnetic artifacts from adjacent bone. Direct sagittal and field with a spinning motion. These spinning pro- coronal MR images are very useful in localizing tons can be flipped to a right angle, to 180° or to lesions and demonstrating small structures such as angles in between by a radio-frequency pulse, after the optic chiasma and tracts, intracannilicular which the protons will return to their resting vestibular nerve and pituitary gland.83-85 position. In so doing they give off a signal captured The spinal cord, pons and medulla can be seen by the radio-frequency coil. These impulses are separate from the surrounding thecal sac and then processed to form an image by a mathematical cerebral spinal fluid.86 Sagittal sections show the technique known as the Fourier transform." whole length of the cord for the diagnosis of The exponential decay of the spinning protons cerebellar tonsil herniation, syringomyelia, intra- produces a variety of signals depending on the medullary tumours and disc herniation and sequence of radio-frequency pulses. If the proton prolapse. In the vast majority of cases contrast decay is a simple repolarization or free induction will not be needed where MRI is decay, the signal then represents only proton available,87'88 and is the method of choice in density. After a 900 pulse followed by a 1800 pulse, evaluating spinal trauma.89'0 the signal is captured from the spin-echo. Differing The bone marrow is another area where MRI relaxation properties of tissues and fluids can be exhibits new imaging information.91'92 Although calculated to produce different images. If the the surrounding bone has no signal- 'signal void'

interval between 90° pulses (repetition time or TR) the imaged bone marrow reflects its changing copyright. is short and the time between the measured echoes pattern due to aging. In children and young (SE or spin echo) is also short, T, relaxation times adolescents active red marrow in both the will be recorded, whereas if these times are long peripheral and axial skeleton has low signal inten- then T2 relaxation times are produced.76 sity on T,-WI, gradually changing to high intensity T, and T2 images are markedly different. In in the peripheral skeleton as fatty marrow broad terms, T, or spin-lattice images have more supervenes. The femoral epiphysis and greater anatomical detail, while T2 or spin-spin images trochanter have high signal intensity in have considerably more contrast between different adolescents, with the shaft and neck of femur http://pmj.bmj.com/ tissues, particularly between normal and abnormal changing to high intensity on T,-WI in the middle soft tissues. Fat and recent haemorrhage are white aged and elderly. The spine is of low intensity in or of high signal intensity (SI) on T1-weighted children apart from the horizontal area around the images (T,-WI) and medium to high signal inten- basivertebral veins. With age this region changes to sity on T2-WI. Cysts or old haemorrhage have low low intensity, while the rest of the vertebral body SI (black) on T,-WI and white on T2-WI. Bone and develops spotty or patchy areas of high intensity. gas produce no signal (signal void).77 Similar changes occur in the skull: the frontal and Recently new sequences have been developed occipital regions develop intense signals and the on September 28, 2021 by guest. Protected that change the signal intensities or markedly alter parietal region lagging with mixed low and high the scanning times, hence the large number of intensity areas, until in middle age the whole acronyms - STIR, GRASS, FLASH,78'79 FAST, cranial vault has high intensity signals on TI-WI. FISP, SPEED and RARE,80 to mention some. It is difficult to detect diffuse bone marrow STIR,8' or short time recovery, is of interest as the infiltration in childhood but in adults, particularly fat signal is suppressed showing a medium signal in the elderly, the replacement of fatty marrow by intensity, unlike T, and T2-WIL With GRASS, or carcinoma metastases, lymphoma or myeloma is gradient-echo recalled acquisition in a steady state, readily demonstrated, unlike isotope bone imaging a gradient radio-frequency pulse is followed by where lymphoma and myeloma cannot usually be small angle pulses to produce images within breath- shown. holding times. However, various artifacts can MRI is being used extensively in orthopaedic occur with these sequences such as black margins practice for a variety of conditions ranging from and even pseudo-lesions. With RARE80 (rapid ligamentous and meniscal trauma,93 osteonecrosis, acquisition relaxation enhanced) sequences pro- especially avascular necrosis of the hip94'95 and duce T2-WI much more rapidly, 16 images within 2 scaphoid, and capsular degeneration of the minutes. shoulder,' to the bone bruise that is not visible on Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from 340 L. KREEL plain radiographs or on CT. Orthopaedic implants the electrons with the external field, changing the such as hip prostheses are not a contraindication for field around the nucleus, that produces 'chemical MRI.97 There have been no reports of unsafe tissue shift' forming the basis of MR spectroscopy. heating nor has there been any evidence of torque. MRS is obtained with apparatus similar to MRI While there is focal loss of signal and some but requires an extremely homogenous external distortion may occur, CT has considerably more magnetic field. In a similar way brief pulses of artifacts in the form of radiating streaks from radio-frequency excite the nuclei.'07 This is fol- implants. MRI in the presence of orthopaedic lowed by a period of signal acquisition. The implants thus has a distinct advantage over CT. acquired signal obtained by free induction decay Soft tissue tumours are more clearly delineated (FID) is Fourier transformed to produce a spect- than on CT.98 rum in which chemicals in solution form narrow MR images of the heart and great vessels are peaks. The area under the peak depends on the truly remarkable,' showing even more than sono- number of nuclei detected. The position in the graphy but at considerably more cost. The major spectrum identifies each particular metabolite. A advantage is that chronic obstructive airways broad range offrequencies is generated by MRS to disease is not an insuperable barrier nor are study nuclei with very differing resonance frequen- calcified costal cartilages or ribs. MRI has the cies, including 3'P to detect ATP, PCr and Pi- advantage over CT as contrast medium is not inorganic phosphorous as an indicator of pH; 'H required and the heart and great vessels can be measures lactates; '3C is used for glycogen, some shown directly from any angle. lipids and compounds using '3C-labelled MRI displays pericardial, myocardial and val- metabolites. 0' vular disease'" as well as abnormalities ofthe great Cardiac and voluntary muscle can be analysed vessels with the aid of the cardiac and respiratory with MRS. During muscular contraction there is a gating.'0' volume and ejection fractions can rapid fall in phosphocreatine (PCr), a rise of Pi and be determined and areas of dyskinesia can be an acid shift of the Pi peak. Only when PCr is visualized.'02 Cardiologists and cardioradiologists virtually depleted does the ATP peak change,

now have the possibility of discarding catheter indicating a single pool of PCr (CK)."'9 Similar copyright. examinations except for coronary angiography and studies have been done on brain,"0 kidney, liver angioplasty. Similarly, contrast angiography may and bowel. MRS of malignant tumours suggests it become superfluous as modifications to MR may be useful in monitoring the effects of treat- angiography produce more detailed images of the ment, providing an early indication of its efficacy"' vascular system. At present the arterial system is and possibly earlier diagnosis of the presence of shown as having high intensity signals and appears malignancy from blood samples."2 This has not as similar to conventional contrast angiography.'03 yet been achieved.

With this MRI technique stenoses are exaggerated http://pmj.bmj.com/ due to distal turbulence and severe stenoses cause poor flow contrast. These artifacts can be elimi- Nuclear medicine nated by using 'black blood' MR angiography where the proton signals appear hypointense."04 Two separable aspects are integrated to form an MRI also has a place in the diagnosis of pelvic image in NM, the recording device and emission of masses during pregnancy if further information is radiation from within the body. Recently, as in required following sonography.'05 other scanning methods, computers have been added, increasing the resolution of the system for on September 28, 2021 by guest. Protected the detection of smaller lesions and to perform Magnetic resonance spectroscopy metabolic studies, particularly of the brain. Over decades the hand-held scintillation detector In vivo biochemical analysis of tissues can be has been replaced by the rectilinear scanner and obtained from nuclei with an odd number of later by the Anger gamma camera. A gamma protons or neutrons ('H, 3'P, 13C, 23Na and '9F) that camera consists of discrete photomultipliers each exhibit nuclear magnetic resonance (NMR), receiving the impulses from the sodium iodide because the resonance frequency of a particular crystals of the scintillation detector to convert the nucleus is affected by its chemical environment in a gamma photons from radionuclides within the way that allows it to be measured.'06 Thus, the body into electrical impulses that are then recorded NMR signal depends on the gyrometric ratio ofthe individually, forming dots on a video monitor. The nucleus and on the intensity of its magnetic field. image can also be recorded on radiographic film. The magnetic field of the nucleus in turn depends In emission CT an injected radionuclide is the on the external magnetic field applied by the internal source ofradiation, unlike X-ray CT where magnet as well as the surrounding electrons and the the source of radiation is an external beam, i.e. electrons of adjacent atoms. It is the interaction of transmission CT. Single photon emission com- Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from MEDICAL IMAGING 341

puted tomography (SPECT) images are produced enzymes. by a single- or dual-headed camera that rotates Biocompatible MoAbs have also now been about the body through either 180° or 360° using developed to locate tumours by being tagged with computers to reconstruct an image in the axial 99mTc, 131I, 123I and "'In. 9`'Tc is favoured because plane. Dual-headed cameras produce better quality larger doses can be used, allowing for earlier images; recently a 3-detector gamma camera (IGE imaging with conventional gamma cameras (4-8 neurocam) has been evaluated."3 hours). "'In has a longer half-life of 2-3 days, SPECT"4 effectively provides greater resolution required when there is slow clearance of the and therefore more detailed information using antibody from the vascular spaces and interstitial conventional radionuclides based on 9'Tc (tech- tissues. netium pertechnetate), 123i, 1311 and "'In (indium) Radioimmunochemistry, as it is now called, has such as IMP (123I p-iodoamphetamine) and HMPAO been used successfully in imaging metastases from (99'Tc hexamethylpropylene amine oxime) for melanoma and colorectal and ovarian carcinoma, cerebral metabolic studies. and in the diagnosis of primary ovarian cancer, as Positron emission tomography (PET),"'' on the well as in infections and vascular thrombi.'21"122 The other hand, is based on a quite different principle. procedure is safe with a high degree of specificity The process relies on positron decay to form for the detection and staging of disease whether high-resolution NM images. A cyclotron is the benign, malignant, infective or metabolic. Recently energy source to produce short half-life radio- systemic amyloidosis has been imaged; the deposits nuclides. As their unstable nuclei decay to a more were rapidly and specifically localized with 123I- stable state they emit positively charged electrons labelled serum amyloid component.'23 As is to be (positrons) and when these positrons combine with expected, the detection rate of lesions shown by an electron their mass is converted into electro- radioimmunochemistry can be increased by imag- magnetic radiation by 'annihilation' producing a ing with SPECT pair of high-energy photons (511 KeV). These two annihilation photons are emitted in opposite direc-

tions, falling simultaneously on detectors on Digital subtraction angiography (DSA) copyright. opposite sides of the patient. In fact there is a ring of detectors to create an axial tomographic image The advent ofscanning ofall forms has diminished of 1-2 cm somewhat similar to MRI and CT. PET the diagnostic role of angiography but it remains has been used mainly to study brain metabolism, essential for interventional procedures to correct or blood flow, tissue pH and cerebral blood volume ameliorate vascular pathology and also as a road using positron-emitting isotopes such as '3C, '3N, map, particularly in transplant surgery. Angio- 15o, 18F and 68Ga. graphy with DSA is safer and more rapidly per- Both SPECT and PET can detect and localize formed. 124 http://pmj.bmj.com/ diffuse and focal cerebral abnormalities producing The principles of subtraction radiography were metabolic and perfusion defects through abnormal described by Ziedses des Plantes more than half a isotope uptake."6 A variety ofdiseases"7 have been century ago. A separate image is produced of the studied including Alzheimer's, Pick's, Creutz- difference between two radiographs by covering feld-Jacob, schizophrenia"8 and cerebrovascular one radiograph with the diapositive of the other, disease where regional blood-flow changes have now readily accomplished by computation with been noted. Perfusion asymmetries appear to be

digitized images. Immediately prior to the contrast on September 28, 2021 by guest. Protected more marked in presenile than in old-age onset medium being injected a baseline image is taken. dementias and greater in the more severely affected. The digital data ofthis image is subtracted from the Epileptic foci can be detected but for accurate subsequent images as they appear on the monitor. pre-operative localization the PET image must be A hard copy of the most diagnostic image is then integrated with MRI or CT 3-D reconstructions if processed on radiographic film and/or stored on surgery is required for uncontrollable seizures.119 disc or tape. The development ofnew radionuclides increased The aorta, pulmonary vessels and large arterial the scope and potential of NM imaging. Outstand- branches, including the carotids, vertebrals, cere- ing among these are the isotopic monoclonal bral, renal and mesenteric branches, can be shown antibodies (MoAbs) that can target specific tissues from an intravenous injection into the superior for both diagnosis and treatment. "'Indium- vena cava or into the right atrium.'25 Intravenous labelled antimyosin (Myoscint) for myocardial DSA has the obvious advantage ofnot requiring an necrosis was the first to be used in clinical practice arterial puncture but it does need a large volume of for diagnosis, prognosis and complications and to concentrated contrast medium. Furthermore there evaluate therapy.'20 It is especially valuable in must be no movement of the patient during the patients with chest pain without classical ECG procedure, necessitating the use of contrast patterns of myocardial infarction or elevated medium that produces little or no side effects, Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from 342 L. KREEL namely expensive non-ionic contrast medium. Radiological screening programmes If the examination is performed by an arterial catheter, placing the catheter in the aorta, con- Mass screening for gastric carcinoma with barium siderably less and more dilute contrast medium can meal examinations in Japan was used to detect be used. Selective catheterization is usually not early surface lesions. Gastric surgery in those needed and can thus be performed more rapidly patients produced 90% 5-year survivals.'29 The using only local anaesthesia and no or minimal results with mass screening for breast cancer are sedation. With DSA there is considerably less somewhat more controversial, although mammo- radiation and much less film is used. graphy is the only reliable method to detect occult Intravenous DSA for suspected renal artery breast carcinoma. Early detection is said to pro- stenosis has the further advantage that a high- duce a 30-70% reduction in the chance of dying quality film of the kidneys and ureters can be from the disease.'30 There is also no doubt that obtained following the arteriogram by taking a can detect 90% of breast cancers, single overcouch film ofthe abdomen and using the and of these 60% are infiltrating but less than I cm contrast medium of the arteriogram to provide an or are non-infiltrating tumours, compared with intravenous urogram.'26 only 6% found by physical examination.'3"' 32 Yet the evidence published by enthusiasts has been criticized, suggesting that, in fact, there is no hard Non-ionic contrast medium evidence that breast cancer screening reduces mor- tality. '33 The water soluble iodine contrast agents for intra- Although the breast is sensitive to X-rays there is venous and intrarterial studies are remarkably safe no definite evidence of an increased risk of car- considering the amount of contrast medium in- cinoma resulting from breast screening. Never- jected per examination, varying between 20 and theless a film/screen combination providing the 50 g.'27 Fatalities have been reported as occurring lowest exposure must be used but without sacri- in 1 in 40,000 patient injections but in a recent ficing detail. This can be achieved with green

report there was not a single death in some 350,000 sensitive film that has a double-sided emulsion and copyright. contrast studies. Patients with heart disease are the new flat orthogonal silver halide grains. particularly at risk of a severe reaction as well as Densitometry is now being suggested as screen- those with asthma and allergy. A history of ing for osteoporosis to identify women at risk who previous contrast reaction is especially important would benefit from hormone replacement or eti- in this regard. dronate therapy. The major adverse reactions are related to the high osmolality of the ionic contrast agents

affecting red blood cells, capillary endothelium, the Summary and conclusions http://pmj.bmj.com/ blood -brain barrier and haemodynamics. Crenell- ated red cells produced by the high osmolality There is now a wide choice of medical imaging to cannot pass through capillaries resulting in anoxia, show both focal and diffuse pathologies in various especially damaging in sickle-cell disease. Capil- organs. Conventional radiology with plain films, laries become weakened, resulting in interstitial fluoroscopy and contrast medium have many ad- oedema, and damage to endothelium allows con- vantages, being readily available with low-cost trast medium to enter brain cells. Local vasodilata- apparatus and a familiarity that almost leads to tion tends to cause a feeling ofwarmth, and pain is contempt. The use of plain films in chest disease on September 28, 2021 by guest. Protected not infrequent. General vasodilatation can cause and in trauma does not need emphasizing, yet there hypotension and collapse, and hypervolaemia are still too many occasions when the answer causes further myocardial strain. obtainable from a plain radiograph has not been Non-ionic contrast agents are relatively free of available. The film may have been mislaid, or the these effects, essential for contrast phlebography examination was not requested, or the radiograph and for intravenous DSA in patients with heart had been misinterpreted. The converse is also quite disease and asthma; they are also recommended in common. Examinations are performed that add diabetes and renal disease. nothing to patient management, such as skull films The only disadvantage of non-ionic contrast when CT will in any case be requested or views of medium is the higher cost,128 some 3-4 times the internal auditory meatus and heal pad thick- greater in Europe but about 10 times greater in the ness in acromegaly, to quote some examples. USA where the cost of ionic contrast agents is Other issues are more complicated. Should the considerably less. Non-ionic contrast agents are patient who clinically has gall-bladder disease have unlikely to become much cheaper in the immediate more than a plain film that shows gall-stones? Ifthe future because of the complexity of the production answer is yes, then why request a plain film if process. sonography will in any case be required to 'exclude' Postgrad Med J: first published as 10.1136/pgmj.67.786.334 on 1 April 1991. Downloaded from MEDICAL IMAGING 343

other pathologies especially of the liver or pan- Just when contrast agents and angiography have creas? But then should cholecystography, CT or become safe and are performed rapidly, they are be added for confirmation? Quite being supplanted by scanning methods. They are clearly there will be individual circumstances to now mainly used for interventional procedures or indicate further imaging after sonography but in of pre-operative 'road maps' and may be required the vast majority of patients little or no extra even less in the future as MRI angiography and information will be added. Statistics on accuracy Doppler techniques progress. and specificity will, in the case of gall-bladder MRI will almost certainly extend its role beyond pathology, vary widely if adenomyomatosis is the central nervous system (CNS) should the considered by some to be a cause of symptoms or if equipment become more freely available, especially sonographic examinations 'after fatty meals' are to orthopaedics. Until then plain films, sonography performed. or CT will have to suffice. Even in the CNS there are The arguments for or against routine contrast conditions where CT is more diagnostic, as in urography rather than sonography are similar but showing calcification in cerebral cysticercosis. the possibility ofcontrast reactions and the need to Then, too, in most cases CT produces results limit ionizing radiation must be borne in mind. comparable to MRI apart from areas close to bone, These diagnostic strategies are also being structures at the base of the brain, in the posterior influenced by their cost and availability; purely fossa and in the spinal cord. pragmatic considerations are not infrequently the Scintigraphy for pulmonary infarcts and bone overriding factor. metastases and in renal disease in children plays a Non-invasive methods will be preferred, partic- prominent role and its scope has increased with ularly sonography as it is far more acceptable by new equipment and radionuclides. Radio- not being claustrophobic and totally free of any in particular is likely to known untoward effects. There is another quite expand greatly not only in tumour diagnosis but different but unrelated aspect. The imaging also in metabolic and infective conditions. Whether methods, apart from limited exceptions, cannot the therapeutic implications will be realized is more

characterize tissues as benign or malignant, problematic. The value of MRS and NM for copyright. granulomatous or neoplastic; cytology or histology metabolic studies in clinical practice is equally usually provides the answer. Sonography is most problematical, although the data from cerebral commonly used to locate the needle tip correctly activity are extremely interesting. for percutaneous sampling of tissues. Frequently While scanning has replaced many radiographic sonography with fine needle aspiration cytology or examinations, has had a similar effect biopsy is the least expensive, safest and most direct on barium meals and to a lesser extent on barium route to a definitive diagnosis. Abscesses can be enemas. The combined visual/sonographic endo-

similarly diagnosed but with needles or catheters scope is likely to accelerate this process. http://pmj.bmj.com/ through which the pus can be drained. There is no doubt that over the last 2 decades The versatility and mobility of sonography has medical imaging has changed the diagnostic pro- spawned other uses, particularly for the very ill and cess, but its influence on the outcome of disease immobile, for intensive therapy units and for the other than infections is less certain and probably operating theatre, as well in endosonography. The undefinable. Data concerning the comparative appointment of more skilled sonographers to the efficacy in terms ofpatient outcome for each of the National Health Service could make a substantial imaging techniques would be of considerable contribution to cost-effective management of hos- interest and a great help in determining diagnostic on September 28, 2021 by guest. Protected pital services. strategies.

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