Diagnostic Imaging of Renal Artery Stenosis

Home , Pyelogram, Renal artery stenosis

Journal of Human Hypertension (1999) 13, 595–603  1999 Stockton Press. All rights reserved 0950-9240/99 $15.00 http://www.stockton-press.co.uk/jhh REVIEW ARTICLE Diagnostic imaging of renal artery stenosis

F Aitchison and A Page City Hospital NHS Trust, Birmingham, UK

Many different imaging modalities and techniques have enzyme inhibitors, intra-venous and intra-arterial cath- been used in the past for the diagnosis of renal artery eter angiography, computed tomographic angiography stenosis. The ideal accurate, non-invasive and inexpen- and magnetic resonance angiography. At present intra- sive diagnostic test has yet to be found but there have arterial catheter angiography remains the ‘gold stan- been promising developments during the last few years. dard’ test. Magnetic resonance angiography is currently In this review we consider in detail the use of the follow- of limited availability in the United Kingdom but is the ing tests for the diagnosis of renal artery stenosis; intra- most promising new development because it is a non- venous urography, B Mode ultrasound, Doppler ultra- invasive test which can be used to obtain both anatom- sound, renal scintigraphy with angiotensin-converting ical and functional information.

Keywords: diagnostic imaging; renal artery stenosis

Introduction the non-invasive tests have previously had only a moderate level of sensitivity and specificity for RAS. Renal artery stenosis (RAS) remains the commonest Therefore these non-invasive imaging methods were curable cause of hypertension and is a significant used as screening tests to select those patients con- cause of end-stage renal failure. In the young hyper- sidered at sufficiently high risk of the disease to jus- tensive patient a diagnosis of RAS caused by fibro- tify proceeding to catheter angiography for definite muscular dysplasia is unusual but must be con- diagnosis. It is hoped that newer non-invasive tech- sidered in the appropriate clinical circumstances. In niques such as magnetic resonance angiography will the United Kingdom there is a much larger group of combine both screening and definitive diagnostic patients with RAS caused by atherosclerotic disease. 1 roles. In this review we will discuss in detail the A recent paper in this journal considered whether diagnostic tests which have previously been used it is worthwhile to diagnose atherosclerotic renal for RAS and those which are currently being artery stenosis (ARAS). The authors concluded that developed. patients with renovascular hypertension caused by ARAS should only be treated when there is true drug resistant hypertension and that the results of Imaging methods treatment for ARAS to halt progression of ischaemic Intravenous urography renal failure are promising. The intravenous urogram (IVU) was first modified Much attention during the past 30 years has for use as a screening test for RAS nearly 40 years focused on the need to find an accurate, inexpensive ago.2,3 It was thought that by taking X-ray films at 1, and non-invasive diagnostic test for RAS. Radiology 2, 3, 4 and 30 min post contrast injection in addition has evolved rapidly during this time. New, non- to the conventional films at 5 and 15 min that cer- invasive diagnostic methods which can be used for tain features could be identified which would indi- RAS imaging such as ultrasound, computed tom- cate renovascular hypertension. These features were ography and magnetic resonance imaging have been believed to be related to reduced arterial blood flow developed. Interventional radiological techniques and hyperconcentration of urine in the affected kid- have been refined to use a lower radiation dose and ney. The specific appearance which were sought smaller calibre catheters. In addition, non-surgical were disparity in renal length, delayed calyceal treatments such as renal angioplasty and renal artery appearance and hyperconcentration of contrast on stenting have become possible. delayed films. At present conventional arterial catheter angiogra- The largest series to favourably assess the use of phy is the ‘gold standard’ diagnostic test for RAS. the IVU in the diagnosis of RAS was the prospective, However, it is an expensive and invasive procedure multi-institutional Cooperative study published in associated with a measurable morbidity. In general, 1972.4 However, the authors also concluded that the IVU was not able to distinguish between those Correspondence: Dr F Aitchison, Department of Radiology, City patients who would respond favourably to surgical Hospital NHS Trust, Dudley Road, Birmingham B18 7QH, UK intervention and those who would not. Received 24 February 1999; revised and accepted 30 April 1999 During the next decade several papers were pub- Diagnostic imaging of RAS F Aitchison and A Page 596 lished which questioned the use of the IVU in the renal length and volume in normal subjects over 70 diagnosis of renovascular hypertension. The most years of age12 which is believed to be due to a influential of these was published by Thornbury et decrease in the number of glomeruli and a reduction al.5 This was a large retrospective study from a sin- in the mass of the juxtamedullary nephrons.13 The gle institution and the authors found a true positive other problem is that a reduction in renal length can rate for the diagnosis of RAS (greater than 50% be caused by many other chronic renal parenchymal stenosis) of only 58.4%. In addition they performed diseases. Therefore measurement of renal length is a re-analysis of the Cooperative study data using a both a relatively insensitive and non-specific test more widely accepted, lower estimate of the inci- for RAS. dence of reno-vascular disease in the general popu- Although it is not a good diagnostic test for RAS lation than the 10% used in the original paper. The it is possible to justify the use of B Mode US in the conclusion was that when used as a screening test general investigation of hypertensive patients to for the detection of RAS (greater than 50%), the exclude other renal disease, such as polycystic dis- Cooperative study data showed a true positive rate ease, chronic pyelonephritis and hydronephrosis. for IVU of 78%. Doppler US is a more complex technique, which In a later review it was calculated that the prob- has been developed during the past two decades to ability of renovascular hypertension in the general demonstrate patterns of blood flow. Recently, hypertensive population only increased from 1% to Doppler US has become much more widely avail- 5% when the patient had a positive IVU examin- able, easier to use and more accurate due to techno- ation. This was concluded to be insufficient evi- logical advances. It now has a well-established place dence for most clinicians to consider referral for in other areas of vascular imaging. There has been catheter angiography.6 very considerable interest in Doppler US as a diag- The IVU involves the use of intravenous iodinated nostic technique for RAS because of its non-invasive contrast material with the small associated risk of nature and relatively low cost. However, despite contrast reaction. The rare adverse effects are extensive study the role of Doppler US in the screen- diverse including urticaria, nausea and vomiting ing, diagnosis and follow-up of RAS still remains and anaphylactoid reactions such as hypotension unclear. and cardiac arrest.7 Iodinated contrast material is The basic principle of US is the use of the reflec- potentially nephrotoxic in diabetics and those with tion of an inaudible sound beam by surfaces at dif- pre-existing renal impairment8 and the IVU is asso- ferent depths within the body to create a map of ciated with a significant effective radiation dose structure. Doppler US depends on detecting a (equivalent to more than 200 chest X-rays). The IVU change in the frequency of the ultrasound beam is not an invasive test but when used for detection when the surface is a moving particle such as a of RAS results have been disappointing. The IVU is blood cell (Doppler effect). A complex mathematical no longer considered an appropriate test in the process is then used to convert this information into investigation of suspected RAS.9 a map of flow in the area under assessment. Both the direction and velocity of flow are indicated by a colour code on the US screen. In addition, a detailed Ultrasound quantitative analysis of the waveform pattern of the Conventional B Mode ultrasound (US) allows blood flow in a smaller area such as a single vessel detailed assessment of the anatomy of the kidney can be obtained as a graph. and accurate measurement of renal size. It is an eas- The earliest studies of Doppler US for RAS diag- ily performed, inexpensive procedure which is well nosis were a direct development of the technique tolerated by patients. Although it was long accepted which had already been successfully used to detect that a reduction in renal size was associated with stenosis in carotid arteries. This depended on find- RAS there was a paucity of formal reports on this ing an increase in blood velocity at the stenosis site subject in the literature for many years. This was and turbulent flow just beyond the abnormal seg- due to the difficulty in performing natural history ment of the main renal artery. Various criteria that studies based on follow-up using invasive func- related to the peak systolic arterial velocity and ratio tional tests or arteriography. With the development of renal artery to aortic peak systolic velocity were of B Mode and Doppler US, non-invasive assess- proposed for the detection of a haemodynamically ment of the natural history of RAS became possible. significant stenosis.14–16 These studies demonstrated that over 20% of Initial reports were encouraging, however during patients with greater than 60% stenosis of the main the next few years a wide range of accuracies for renal artery will sustain a reduction in renal length diagnosing or excluding RAS were reported17 and it of at least 1 cm during a 2-year follow-up period.10,11 soon became clear that there were significant diffi- There are three main problems with the use of a culties in the study of the main renal artery. The measurement of renal length as a diagnostic test for technique was time consuming and operator depen- renal artery stenosis. The first is the wide variation dent. Inability to fully demonstrate technique was in normal renal length between subjects.12 A confi- time consuming and operator dependent. Inability dent diagnosis of abnormality is only possible if a to fully demonstrate the vessel was reported in up to previous US measurement is available to prove a 40% of studies.18,19 This was due to various factors reduction in length or where there is a marked including patient obesity, respiratory motion, trans- reduction in renal length in the later stages of the mitted cardiac or aortic pulsation or to overlying disease. The second difficulty is the reduction in bowel gas. If the vessel followed a tortuous course Diagnostic imaging of RAS F Aitchison and A Page 597 there was difficulty in demonstrating its entire been used to quantify the changes described above. length with confidence. There was further doubt Sensitivity of up to 92% and specificity of up to about the validity of the technique when studies 95% for detection of severe RAS (greater than 70%) showed accessory renal arteries present angiograph- have been reported.29–31 Generally, it was found that ically in 14–24% of kidneys and only 0–4% of these moderate renal artery narrowing (50–70%), did not were shown with Doppler US.18–20 produce convincing Doppler waveform changes in Doppler US examination of the intra-renal arteries the intra-renal vessels. These studies also found that is much less technically challenging than assess- peripheral renal Doppler studies could not always ment of the main renal artery. In 1986 Handa et al21 reliably distinguish severe stenosis from complete first observed that changes occur in the waveform occlusion.17 pattern of the intra-renal vessels beyond a renal In an effort to improve detection of RAS, it was artery stenosis. The waveform changes in the intra- suggested that an oral angiotensin-converting renal vessels were observed to be of two basic types enzyme (ACE) inhibiting drug be added during the (Figure 1). The first type manifested as a slowed rise study. A similar approach had been previously used to peak systolic velocity (‘pulsus tardus with some success in Nuclear Medicine examin- phenomenon’) and loss of the normally occurring ations (see later for detail). In two studies, the use early systolic velocity peak.21–24 The second type of an oral ACE inhibiting drug during the intrarenal was a lowering of the renal resistive index (peak sys- Doppler US study was shown to improve the detec- tolic velocity/maximum end diastolic velocity).25,26 tion of moderate RAS (50–70%).32,33 However, the These changes were shown to be result of a complex reproducibility of these results is uncertain and the interaction between the resistance to flow and the mechanism of the effect on the Doppler waveform compliance of the post-stenotic vessel wall.27,28 is the subject of considerable debate. There have been many studies published on the As discussed above, the intra-renal waveform use of intra-renal Doppler US to detect RAS. Results alteration in RAS is partly dependent on vascular have been variable and a number of indices have compliance. Therefore it is possible that a false positive result could be produced in a patient with reduced wall compliance such as occurs in atherosclerotic disease or with the normal loss of arterial wall elasticity during aging. This is one reason why research has been directed to improve visualisation of the main renal artery. The use of the main renal artery for assessment would also allow the use of the original, less complicated technical parameters for the detection of stenosis. During recent years, intravenous contrast media have been introduced for use with ultrasound.34 These are of low toxicity and improve Doppler signal detection from both the main renal artery and intra-renal vessels. There are several types of intravenous contrast media under investigation. One of these is a galactose micropar- ticle suspension containing microbubbles (Levovist, Schering, Berlin, Germany) which has been evaluated with promising results.35 However, at present the use of Doppler US for the diagnosis of RAS is still perceived to be dependent on operator skill and experience. The technique has not gained widespread acceptance as a robust screening test for RAS in the general hypertensive population. It may have a role in more selected patient groups or to monitor treatment results in individual patients.

Renal scintigraphy with angiotensin-converting enzyme inhibitors Renal scintigraphy involves the intravenous (i.v.) injection of complex molecules bound to radioactive markers (radiopharmaceuticals). The two most commonly used agents in imaging RAS are diethylenetri- amine pentaacetic acid (DTPA) and mercaptoacetyl- Figure 1 Intra-renal Doppler measurements in patient with pro- triglycine (MAG-3). Once these are attached to the ven left sided RAS. (a) Normal right kidney intra-renal Doppler isotope 99mtechnetium (labelled) they are given to signal, (b) abnormal left kidney intra-renal Doppler signal with loss of the early systolic velocity peak and ‘flattening’ of the wave- the patient as an i.v. bolus and can be detected with form. (By kind permission of Dr G Baxter, Western Infirmary, a gamma camera. The labelled DTPA or MAG-3 is Glasgow). filtered by the kidney and excreted in the urine. Diagnostic imaging of RAS F Aitchison and A Page 598 Dynamic counting of radioactivity, with regions of administered i.v. about 10–15 min prior to the radi- interest over the kidneys, allows us to study renal opharmaceutical.36 Typically 25–50 mg of oral cap- function. A computer creates the renogram curves topril is used.38–40 Profound hypotension may follow which are processed enabling assessment of the split the first dose of ACE inhibitor and therefore the renal function, the time-to-peak-counts and the blood pressure is monitored regularly and any hypo- renal parenchymal transit time. tensive episodes treated appropriately. Haemodynamically significant RAS causes a In patients with RAS the abnormality on the reno- reduction in renal perfusion. There is then acti- gram depends on the radiopharmaceutical used. vation of the renin-angiotensin system, resulting in DTPA is exclusively excreted by glomerular fil- elevated levels of angiotensin II (AT II). The glom- tration and thus in the presence of RAS there is erular filtration pressure and rate are maintained by decreased uptake and excretion on the baseline AT II mediated efferent arteriolar vasoconstriction. study, more marked on the ACE inhibition study. However, AT II also causes systemic hypertension MAG-3 is predominantly excreted by the renal by increasing the tubular resorption of sodium, both tubules and thus significant stenosis results in a directly and indirectly via increased aldosterone delayed transit time and persistent cortical activity. secretion, and by raising the total peripheral resist- The sensitivity and specificity of renal scintigra- ance via generalised arteriolar vasoconstriction. phy in the diagnosis of RAS are very variable. The ACE inhibitors, which block the synthesis of AT II results depend not only on correct patient selection from angiotensin I, can theoretically unmask the but also on the radiopharmaceutical used and the activation of the renin-angiotensin system which method of interpretation of the renographic changes. may be detected by scintigraphy. Reported figures range from 34% to 100% for sensi- The basis of all methods of renal scintigraphy for tivity and 72% to 95% for specificity.37,38,41 Some the detection of RAS is to demonstrate delayed and authors rely on post ACE inhibition renograms decreased total activity over the affected kidney only40 and others feel that overall diagnostic accu- (Figure 2). There is no consensus on the most effec- racy has not been improved since the addition of tive protocol for the use of renal scintigraphy to ACE inhibitors.39 detect RAS.36,37 Prior to the examination patients are The usefulness of renal scintigraphy as a diagnos- usually fasted but allowed free fluids to maintain tic test for renal artery stenosis has recently been good hydration. Both baseline and/or ACE inhi- questioned.39 Its negative predictive value is 72% bition scintigraphy can be performed. If both studies and in one series a third of patients with proven are used a suitable time interval must elapse before renovascular hypertension had negative ACE inhi- the ACE inhibition scintigraphy is performed to bition scintigraphy.42 The use of post-captopril renal allow clearance of the isotope. Patients are required scintigraphy for the prediction of response to inter- to discontinue any oral diuretic and ACE inhibitor vention in RAS has shown more consistent results. therapy for up to a week prior to baseline studies. A positive predictive value of between 90–97%, A variety of commercially available ACE inhibitors with a sensitivity of 87–93% and a specificity of 93– are used for the ACE inhibition study, either taken 100% in predicting the blood pressure response to orally approximately 1 h before examination, or intervention have been found.38,43 Scintigraphic measurement of renal blood flow pre and post ACE inhibitors has also been investi- gated as a diagnostic tool but appears to be of limited value in diagnosing renovascular hypertension or predicting the response of blood pressure to intervention.44 Overall the usefulness of ACE inhibition scintigraphy as a screening test for haemodynamically significant renal artery stenosis is questionable, although a positive result is a strong predictor of those patients who may respond to intervention.

Angiography Angiography involves the injection of iodinated contrast material by the intra-arterial (i.a.) or intravenous (i.v.) route to define a vessel lumen. Conven- tional X-ray film-screen technology was used for many years as the standard technique for visualising and locating the anatomic lesions of renal artery stenosis during angiography.45 Since the introduction of digital equipment most departments now perform digital subtraction angiography (DSA).46 By using DSA all background detail such as the bony Figure 2 Renal scintigram with ACE inhibitor in patient with known left RAS. There is decreased and delayed total activity skeleton is removed from the image by computer. over left kidney (peak counts right kidney Ͼ 8013, left kidney The resultant images show the vessel under study Ͻ 2404). to best advantage and the technique is associated Diagnostic imaging of RAS F Aitchison and A Page 599 with a lower radiation dose than conventional film- the proximal portions of the vessels are often super- screen technology. At angiography, RAS lesions imposed over the aorta despite the use of oblique causing a greater than 50% reduction in the diam- projections. eter of the vessel are assumed to be haemodynami- Intra-venous DSA was introduced in the late cally significant. 1970’s as a less invasive alternative to conventional Intra-arterial angiography is an invasive technique angiography and involves injection of iodinated most commonly involving femoral artery puncture contrast media into the antecubital vein or, follow- under local anaesthesia. Transradial, transbrachial ing insertion of a catheter, into the superior vena or transaxillary approaches are feasible but are asso- cava, inferior vena cava or right atrium.55 Studies ciated with higher morbidity.47,48 A 5 French pigtail have shown that good results can be obtained using catheter is then usually introduced and a flush aor- intra-venous DSA for the diagnosis of RAS with sen- togram is obtained with the tip of the catheter above sitivities and specificities of 88–100% and 90–93% the level of the renal arteries. If necessary, selective respectively.56,57 However, it cannot achieve equiv- renal angiograms are performed with a preshaped alent resolution to conventional angiography and visceral catheter positioned within the origin of the may miss up to 50% of cases of fibromuscular dys- renal artery. Most radiologists now use non-ionic plasia and stenoses in small branch arteries.58 In contrast media49 and a total of approximately 50– addition a larger volume of contrast is needed for 120 mls of contrast are injected via an automatic each series of images increasing the risk of conges- injector. The images are repeated at appropriate tive cardiac failure and nephrotoxicity in patients obliquities to demonstrate the origins of the ves- with pre-existing renal impairment. sels optimally. The procedure is now routinely performed as a Computed tomographic angiography day-case,50 with admission for a 3–6 h post-procedure recovery period.46 Complications due to the Computed tomographic (CT) angiography is an technique include most commonly local haemor- evolving non-invasive method for imaging the renal rhage and haematoma formation, which is seen in vasculature. Unfortunately it does involve the use 15–20%,51 arterial thrombosis and distant emboli, of both ionising radiation and intravenous iodinated sepsis and arterial dissection. Iatrogenic aortic dis- contrast media. However, it has the advantage over section is also reported rarely in elderly.52 Late com- angiography that it also provides anatomical and plications include false aneurysm or more rarely art- pathological information about the kidneys and eriovenous fistula formation at the puncture site.51 adrenal glands. CT angiography can give additional There is a very small additional risk from the con- information about the state of the vessel wall includ- trast media itself, as described in the IVU section ing mural calcification and plaques59 and is less above, although the introduction of low-osmolar costly in time and staff resources than intra- non-ionic contrast media has reduced the overall arterial angiography. rate of complications to approximately 44 cases per In order to evaluate the renal arteries adequately million examinations.53 it is necessary to use a modern spiral (helical) CT Although intra-arterial DSA is now accepted as scanner, rather than conventional CT. Spiral CT is the standard technique for diagnosing RAS (Figure not currently available in all centres. The spiral CT 3) it has its limitations. There is little information on allows the acquisition of multiple contiguous slices the state of the vessel wall and eccentrically located in a ‘block’ during a single patient breath hold. stenoses may not be visible due to the limited num- These images can then be manipulated by computer ber of planes of projection. There is also wide vari- to generate a ‘three-dimensional’ reconstruction ation in the angle of origin of the renal arteries54 and block. In a conventional CT scanner only one image can be obtained during each breath hold. It is extremely difficult for the patient to achieve pre- cisely the same degree of breath hold each time and as a result there is often uneven spacing of images. This creates considerable difficulty in the reconstruction of data. In addition, the intravenous injection of contrast must be maintained during the examination and this would require an unac- ceptably large volume of contrast using conventional CT. The renal arteries are technically difficult to image using CT angiography due to the variation in their number and sites of origin. Although accessory renal arteries can arise as far cranially as the inferior phrenic artery and as far caudally as the median sac- ral artery, almost all arise from the abdominal aorta between the superior mesenteric artery and the aortic bifurcation. There is also considerable excur- sion of the kidneys and renal arteries during respir- Figure 3 Intra-arterial digital subtraction angiogram showing ation and thus an appropriate area needs to be left RAS. scanned during a single breath hold of about 30 sec. Diagnostic imaging of RAS F Aitchison and A Page 600 This area can be determined using a topogram or following localising uncontrasted transverse CT sec- tions.59 It can difficult to obtain adequate diagnostic images due to respiratory or movement artefact in patients with dyspnoea and those unable to lie still. A three-dimensional volumetric acquisition of data is thus performed during a breath hold with simultaneous intra-venous contrast enhancement. Satisfactory renal artery visualisation depends on a high concentration of contrast within the vessel lumen, therefore a large volume (100–150 mls) of non-ionic iodinated contrast media is administered via an automatic injector with a cannula in an antecubital vein. This can be given at a fixed rate, following a fixed delay, depending on the age and weight of the patient.60,61 However, if possible it is preferable to use an individually calculated delay based on the transit time of a preliminary test injection of Figure 4 MR angiogram showing left RAS. Arrow indicates site 15 mls of contrast to optimise contrast enhance- of stenosis. ment.59 The large volume of contrast needed presents an additional risk of nephrotoxicity in those patients with pre-existing renal impairment of the technique for imaging vessels (MRA) is a more although one recent study reported only a transient recent innovation. The image produced in MRI increase of up to 20% in the serum creatinine fol- depends on differences in behaviour between lowing CT angiography.62 tissues when placed in a magnetic field and exposed The diagnostic accuracy depends on the acqui- to repeated radiofrequency pulses. These differences sition and reconstruction parameters used.59 Current are of two main types known as the T1 and T2 relax- opinion differs as to the optimal acquisition proto- ation times. col and the technical factors are often a compromise, There are only a few contra-indications to MRI. being partly dependent on the type of scanner avail- In the past, the technique was often impractical for able.63–65 The computerised analysis of the data patients with claustrophobia because of the need to includes various reconstructions such as shaded- lie in a magnet which had a tunnel like structure. It surface display, maximum intensity projections and was known to induce acute anxiety symptoms in up multi-planar reformatting. These allow detailed to 10% of people requiring abortion of scanning in assessment of the vasculature. It is advisable to approximately 1%.69,70 Improvements in magnet review a combination of reconstructions and orig- design have reduced these problems but it is still inal axial images to accurately grade the stenosis necessary for the patient to lie still during the exam- and to differentiate mural calcification.59,66 ination. Patients with pacemakers and certain ferro- Several studies have compared the accuracy of CT magnetic aneurysm/haemostatic clips, valve angiography with conventional and digital subtrac- replacements and metallic intra-orbital foreign bod- tion angiography. CT angiography is limited in its ies are unsuitable for MRI.70,71 evaluation of small accessory, segmental or intra- There are two basic MRA methods known as time- renal arteries60,61 and may overestimate the degree of of-flight (TOF) MRA and phase-contrast (PC) MRA. stenosis in a small percentage.62,67 However studies These are both flow sensitive techniques and can be have shown that CT angiography is able to visualise acquired in either 2-D or 3-D formats. Breath holding 94–100% of main renal arteries and 78–100% of is preferable but not essential. In TOF MRA the con- accessory renal arteries, depending on the acqui- trast between flowing blood and the adjacent station- sition protocol.60,62,63,68 The technique has recently ary tissue depends on the inflow of fully magnetised reported sensitivities and specificities for detection protons into a preselected volume of saturated of significant (greater than 50%) stenosis of the main stationary tissue.72 PC MRA relies on the phase renal arteries of 100% and 97–98% respectively,60,63 change induced in moving spins as they pass and sensitivities and specificities for detection of through a magnetic field gradient. In addition to ana- significant stenoses in all renal arteries of 88–90% tomical information, PC MRA also allows direct and 97–98%, respectively.60,63,67 quantitative evaluation of flow dynamics with the intensity of flow signal being related to flow velocity.73 In both techniques post processing with Magnetic resonance angiography maximum intensity projections or surface rendering Magnetic resonance angiography (MRA) is emerging is required with areas of stenosis appearing as signal as a promising non-invasive tool to diagnose RAS voids or areas of narrowing. (Figure 4). It has significant advantages over both More recently, intra-venous contrast agents have intra-arterial angiography and CT angiography in been introduced for use with MRA.74 These have that it does not use either ionising radiation or iodi- been found to increase the diagnostic information nated contrast media. Magnetic resonance imaging available, particularly in patients with poor renal (MRI) has been in use for general clinical imaging blood flow. The substances are gadolinium chelates for more than 15 years but the clinical development which are paramagnetic. When injected they induce Diagnostic imaging of RAS F Aitchison and A Page 601 shortening of the T1 relaxation of the flowing blood. the use of MR angiography for both screening and In practical terms this translates as a selective diagnosis will increase during the next decade as increase in the signal intensity in the arterial system, this equipment becomes more widely available. maximally effective during the extracellular first pass of the contrast media. In view of this, precise References contrast media timing is necessary and is usually calculated following a preliminary test bolus. A 1 Eardley KS, Lipkin GW. Atherosclerotic renal artery fixed delay can also be used, although if inappropri- stenosis: is it worth diagnosing? 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The role of urography in unilateral pain and there have been case reports of headache, renovascular disease. 3. Appraisal of arteriography. 4. vomiting and anaphylactoid reaction.70 Arteriographic complications. JAMA 1972; 220: 1195– 1204, 1209–1230. 1972; 221: 368–378. Various MRA sequences have been evaluated. In 5 Thornbury JR, Stanley JC, Fryback DG. Hypertensive comparison with intra-arterial DSA the 2-D TOF urogram: a nondiscriminatory test for renovascular sequence has reported sensitivities and specificities hypertension. Am J Roentgenol 1982; 138: 43–49. of 91–100% and 92–94% respectively for diagnosis 6 Mushlin AI, Thornbury JR. Intravenous pyelography: of greater than 50% stenosis of the proximal por- the case against its routine use. Ann Intern Med 1989; tions of the main renal arteries.75,76 The 3-D TOF 111: 58–70. sequence may allow better detection of accessory 7 Spring DB, Bettmann MA, Barkan HE. Nonfatal renal arteries77 and has sensitivities and specificities adverse drug reactions to iodinated contrast media: of 100% and 89–90% in detecting stenoses greater spontaneous reporting to the U.S. Food and Drug than 60% when compared to DSA.77,78 The 3-D PC administration 1978–1994. Radiology 1997; 204: 325–332. MRA has been shown to be at least as accurate as 79 8 Lautin EM et al. Radiocontrast-associated renal dys- DSA with sensitivities and specificities of 84–94% function: incidence and risk factors. Am J Roentgenol and 91–96% respectively in detecting a greater than 1991; 157: 49–57. 50% narrowing.80,81 The 3-D contrast enhanced 9 RCR Working Party. Making the best use of a Depart- MRA is rapidly gaining acceptance and has a sensi- ment of Clinical Radiology: guidelines for doctors tivity and specificity of 93–100% and 92–98% (Fourth Edition). The Royal College of Radiologists: respectively in diagnosing stenoses greater than London, 1998, p 61. 50% compared to DSA.81,82,83 10 Guzman RP et al. Renal atrophy and arterial stenosis. The MR imaging sequences and MRA techniques A prospective study with duplex ultrasound. Hyper- available are ever increasing and evolving. A proto- tension 1994; 23: 346–350. 11 Caps MT et al. Risk of atrophy in kidneys with atheros- col involving a combination of sequences appears to 84,85 clerotic renal artery stenosis. Kidney Int 1998; 53: be optimal for imaging the renal vasculature. In 735–742. particular combining the 3-D contrast enhanced 12 Emamian SA, Nielson MB, Pederson JF, Ytte L. Kidney MRA with a 3-D PC MRA and standard T1 and T2 dimensions at sonography: correlation with age, sex images would provide anatomical and pathological and habitus in 665 adult volunteers. 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