2 Computed Tomography (CT) and CT Arthrography
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Computed Tomography (CT) and CT Arthrography 15 2 Computed Tomography (CT) and CT Arthrography Richard William Whitehouse CONTENTS the assessment of acute trauma (e.g. acetabular frac- tures), but MR is considerably better for assessment 2.1 Introduction 15 of soft tissue injuries and tumours. The addition of 2.2 Developments in CT 15 2.2.1 Slip Rings 16 arthrography further increases the specificity and 2.2.2 X-Ray Tubes 16 sensitivity of both MR and CT for articular and 2.2.3 X-Ray Detectors 16 acetabular labral lesions. CT remains essential in 2.2.4 Helical CT (Spiral or Volume Scanning) 16 the assessment of patients in whom MR is contra- 2.2.5 CT “Fluoroscopy” 17 indicated (e.g. due to intracranial aneurysm clips 2.2.6 Data Manipulation 17 2.2.7 Reformatted Images 18 or cardiac pacemakers). CT therefore continues to 2.3 Scan Image Quality 18 have a role in the diagnosis and management of 2.3.1 Internal Metalwork from Fixation Devices 19 many pathologies of the pelvis and hips. Having 2.3.2 CT Number, Hounsfield Units, Window Width decided that CT is an appropriate investigation for and Levels 19 an individual, the precise format of the examination 2.3.3 Radiation Dose Reduction 21 2.4 CT of the Hip and Pelvis 21 will depend upon the suspected pathology and the 2.4.1 Anatomy 21 equipment available. This chapter describes recent 2.4.2 Immobilisation 23 developments in CT scanners, considerations for 2.4.3 Patient Positioning 23 pelvic and hip CT scanning, dose reduction strate- 2.5 Indications 23 gies, CT hip arthrography and CT guided interven- 2.5.1 Trauma 23 tion. The main aim is to outline those considerations 2.5.2 Articular Cartilage 24 2.5.3 Femoral Anteversion 25 that should optimise the images obtained, whilst 2.5.4 Soft Tissues 25 minimising the radiation dose to the patient, what- 2.5.5 Tendons 26 ever CT scanner is used. 2.6 Arthrography 26 2.6.1 Technique 26 2.7 CT Guided Interventions 27 2.8 Conclusion 28 References 28 2.2 Developments in CT A CT image is a Cartesian co-ordinate map of nor- 2.1 malised X-ray attenuation coefficients, generated by Introduction electronically filtered computerised back projection of X-ray transmission measurements in multiple Computed tomography (CT) and magnetic reso- directions through a section of the object in question. nance (MR) imaging are now established methods Those areas where recent developments have been of imaging investigation and both methods continue made include helical scanning, multislice acquisi- to develop. CT remains more suitable than MR in tion and real time CT “fluoroscopy” (Dawson and Lees 2001). These developments have been made on the back of improving technology which includes slip-rings for power and data transmission to and from the gantry, higher heat loading and more rapid R. W. Whitehouse, MD heat dissipation X-ray tubes, high efficiency solid Department of Clinical Radiology, Manchester Royal Infi r- state X-ray detectors, faster data transmission and mary, Oxford Road, Manchester, M13 9WL, UK processing abilities of the electronics. 16 R. W. Whitehouse 2.2.1 emitting light after the X-rays had terminated (after- Slip Rings glow), and other technical problems with respect to the size of the front face of the individual detectors The use of cables to supply power and take data from and the interspace material between adjacent detec- the rotating scanner gantry has now been super- tors have been largely overcome. seded by slip rings. Replacing the cables with slip The ease with which solid state detectors can be rings (large circumference electrically conducting stacked in parallel adjacent channels has facilitated rings) which encircle the X-ray tube path, and trans- the development of multi-slice scanners. These ferring power from the rings to the X-ray tube via scanners can acquire multiple sections simultane- conducting brushes on the X-ray tube gantry, allows ously, which can be separately processed to give the gantry to be continuously rotated in one direc- large numbers of thin sections, or recombined to tion. This has several advantages over the alternat- give fewer thicker sections with lower noise. ing wind up and unwind gantry rotation directions required by continuous cables. Rapid acceleration and deceleration of the gantry are no longer required 2.2.4 yet a faster rotation speed can be achieved giving Helical CT (Spiral or Volume Scanning) shorter scan acquisition times. The time delay between slices need be no longer than that required The requirement for a break in the X-ray emission for table movement in conventional acquisition whilst the table is moved to the next slice position mode and the potential for acquiring continuously was overcome by the development of helical scan- updated X-ray transmission data allows both helical ning. Helical scanning is performed by moving scanning and CT fluoroscopy. the table continuously during the exposure, from the first slice location to the last. Thus a helix of X-ray transmission data through the scan volume 2.2.2 is acquired. To generate a CT image the data from X-Ray Tubes adjacent turns of the helix are interpolated to pro- duce transmission data which are effectively from The development of slip rings resulted in a require- a single slice location (Kalender et al. 1990). This ment for X-ray tubes to have both a higher heat process can be performed at any location within the capacity and a higher maximum tube current, as helix, (except the first and last 180°’s – where there the mAs required for a single slice remained much is no adjacent helix of data for interpolation). In the same but the time in which the slice was acquired this way overlapping slices can be produced without was reduced. As an alternative to higher heat capac- overlapping irradiation of the patient. The relation- ity, more rapid heat dissipation from the tube has ship between the X-ray fan beam collimation and the been developed by one manufacturer. In addition, table movement per rotation of the gantry is called for helical scanning continuous X-ray output for up the pitch ratio. Extended or stretched pitch scans to 60 s may be required. The disadvantage of these are performed with pitch ratios greater than 1. Such X-ray tubes is the increased ease with which high extended pitches can be used to trade off between radiation doses can be given to patients during CT greater scan volumes; shorter scan acquisition times investigations. and lower scan radiation doses. Stretching the pitch ratio to 1.25 has little effect on the image appearances, but pitch ratios greater than 1.5 produce images with 2.2.3 effective slice thickness’ significantly greater than X-Ray Detectors the nominal fan beam collimation thickness. Mul- tislice scanners in particular may use pitch ratios Xenon gas detectors, used in CT scanners for many of less than 1, this increases patient radiation dose years, have a conversion efficiency (X-rays to signal and scan acquisition time but reduces image noise strength) of around 60%, which can diminish fur- and some spiral scanner artefacts. By increasing the ther if the detectors are not maintained. Solid state number of detector arrays (“multi-slice scanner”) crystal detectors may have conversion efficiencies of several interlaced helices can be acquired simultane- nearly 100%, resulting in a 40% reduction in patient ously with the table increment per gantry rotation radiation dose for the equivalent scan appearances. increased proportionately (McCollough and Zink The tendency for solid state detectors to continue 1999). Initial developments in multi-slice scanners Computed Tomography (CT) and CT Arthrography 17 were aimed at reducing individual slice thicknesses, or even 180° gantry rotation datasets. Such “partial but once z-axis resolution is equivalent to in plane scan” images have acquisition times proportionately resolution, further reduction in slice thickness is of shorter than full rotation scans. This can be useful for limited value. Adding further rows of detectors will reducing movement artefacts in selected patients. For then increase the total width of the detector bank. a 0.5 s per rotation scanner, the effective scan acquisi- Whilst offering the potential for faster scan acquisi- tion time will be one quarter of a second (250 ms). If tion, the increasing divergence of the X-ray beam to the gantry continues to rotate and acquire data with- the outer rows of detectors creates a “cone beam” out table movement, continuously updated transmis- geometry for the X-ray beam paths, requiring complex sion data will be collected from which revised images data corrections to reduce artefacts in the resultant can be generated. With extremely rapid processors images. Currently scanners offering up to 64 detector and appropriate reconstruction algorithms, further rows with up to 4 cm total detector width are avail- delay for image reconstruction can be minimised and able. Flat panel detectors, based on the Direct Digital a continuously updated CT image displayed in “near Radiography technology, for CT data acquisition are real time” (Hsieh 1997). Such “CT fluoroscopy” under development. These detectors will allow both imaging can be used for CT guided interventional increased total detector width and variable effective procedures. As with all fluoroscopic procedures care slice thickness. The end result will be a scanner that should be taken to reduce fluoroscopy time to the can acquire the full examination data from a single minimum necessary and to avoid operator irradia- gantry rotation without table movement. Whilst this tion – instruments designed to keep the operators offers significant reduction in acquisition time and hands out of the CT section (Daly et al.