Renal Single Photon Emission Computed Tomography: Should We Do It? E. David Williams Although single photon emission computed t o m o g r a phy (SPECT) imaging has established a place for itself in clinical nuclear medicine for heart and brain studies, its place in renal imaging is not yet clear. Renal SPECT has been subject to limitations imposed by the efficiency of imaging equipment, and has been confined to use with static imaging agents such as technetium99m {~mTc) dimercaptosuccinate (DMSA). SPECT has been used to investigate space-occupying lesions and
anatomical abnormalities, and for quantitative studies of renal uptake and volume. In these areas, it has provided little advantage over conventional imaging, but it has been helpful in individual cases. Highresolution SPECT is a promising new development, which may have applications in detecting and classifying renal scarring. It deserves careful evaluation.
of single photon emisT HEsionTECHNIQUE computed tomography (SPECT) is
distribution of a radiopharmaceutical in the body. This is therefore the route both to better image contrast and hence quality, and to quantify radioactivity distributions. 1 There has been relatively little use of SPECT for dynamic studies, mainly because after administration of the amounts of radioactivity currently considered acceptable for diagnostic use, acquisition of sufficient counts to reconstruct useful images takes about 10 minutes with a single rotating gamma camera. 2 Thus, only very slow dynamic studies are possible. Newer, high-sensitivity equipment may permit dynamic tomographic studies, but no such studies of the kidney have been reported yet. This review is therefore concerned only with "static" imaging, ie, when the amount and location of radioactivity does not change during image data acquisition. Many static imaging investigations in clinical nuclear medicine are concerned with detection of lesions, rather than estimation of whole organ function. Experience to date suggests that SPECT may provide improvements in detection of lesions when looking for defects in a region of fairly uniform uptake, or where the normal pattern of uptake in the region of the lesion is complex. As an example of the latter situation, while planar imaging appears adequate for much of bone scintigraphy, SPECT has been found useful in the skull, lumbar spine, and hip joints. Earlier in the history of SPECT, liver imaging seemed to be a promising area of the former type of application, but more recently radionuclide imaging for detection of lesions in the liver has largely been superceded by ultrasound and computed tomography (CT), which can generally detect smaller lesions and indicate their nature. The principal diagnostic appli-
now well established in clinical nuclear medicine for myocardial and cerebral perfusion imaging. It is indeed the standard imaging technique for these applications in many departments. While SPECT is also used for imaging other organs, such applications tend to be restricted to second-line or specialized investigations. This review concerns the question of whether SPECT can and should be used for renal investigations. This question is primarily concerned with the technical advantages that SPECT may (or may not) have over planar imaging. Three components to the question need to be answered: why should it be used, how should it be applied, and when--to what clinical problems? WHY SPECT? THE PHYSICAL SITUATION
Planar imaging with a gamma camera is the "normal" method used throughout clinical nuclear medicine. It is relatively simple to use, in that for static imaging no processing beyond the display of images is required. It is also readily applicable to dynamic studies. In contrast, SPECT requires more specialized equipment and generally takes longer to acquire, process, and display images. Its use therefore requires special justification. However, emission computed tomography is now seen as the method that can provide more accurate depiction of the
From the Regional Medical Physics Department, District General Hospital, Sunderland, UI~ Address reprint requests to E. David 14711lares, PhD, Regional Medical Physics Department, District General Hospital, Sunderland SR4 7TP, UIC Copyright 9 1992 by W..B. Saunders Company 0001-2998/92/2202-0005505.00/0 112
Copyright9
1992 b y W.B. Saunders
Company
Seminars in Nuclear Medicine, Vol XXll, No 2 (April), 1992: pp 112-121
RENAL SPECT
113
cations of SPECT in recent years have been for imaging perfusion in the brain and myocardium. A common factor in both of these applications is that defects of uptake are sought in a situation where there is fairly uniform uptake throughout the organ, and defects may be obscured by uptake in adjacent tissues. This situation, of a "cold target," is one where improving image contrast can give better lesion detection. A conclusion that may be extrapolated from this is that since the kidney presents a physically similar problem--the renal parenchyma has morphological similarity to the myocardium and lesions also give decreased uptake--it is therefore a potentially suitable organ for application of SPECT with its good image contrast. In addition, whole kidney function is also, and more usually, of clinical interest, and therefore so are methods for improving assessment of function. HOW
lesion contrast and minimizing image noise. Commercial software and terminology may vary so much that specific recommendations are difficult. Therefore, use of a phantom that simulates as closely as possible the size of the patient, the count rate, and the physical detail to be demonstrated is highly recommended. General purpose body section phantoms may well be adequate, although they do not usually provide objects similar to the kidneys. A range of reconstruction filters and display parameters should be tried and the optimum then selected for clinical use. A compromise must be found between factors that give the best contrast, such as a narrow slice width and "sharp" spatial filter, and the opposite (wide slice width, "smooth" filter) which give a smooth, low-noise image. Parasagittal and coronal sections are often much more useful than transverse sections: display of all three planes is therefore recommended. Oblique sections may also be helpful.
Equipment and Techniques The basic principles of SPECT, in which gamma emissions from the subject are gathered over a wide angle and used for a computer reconstruction of section images of the object, are well described in monographs on the subject 3and in textbooks dealing with the physics of nuclear medicine 4 and so will not be repeated here. A rotating gamma camera system in which a standard gamma camera detector is rotated about a horizontal axis through the patient who lies on a narrow couch, has been the most widely used system for data acquisition over the last 10 years. A new, and perhaps highly significant, development has been the introduction of triple gamma camera SPECT systems, which provide a substantial increase in sensitivity. 5 Such systems encourage the use of higher resolution collimators, which are an essential component in attempting detection of smaller lesions. 6 In any application of SPECT, attention to details of technique cannot be overemphasized. Quality control of the equipment, including regular assessment of and correction for gamma camera nonuniformity, is essential, as are mechanical stability of the gantry and correction for center of rotation offset. Appropriate slice thickness, filtering, and display parameters must also be selected with the aim of maximizing
Radiopharmaceuticals SPECT has been applied to renal imaging using static agents only. Technetium-99m (99mTc) dimercaptosuccinate (DMSA) has been most widely used. However, it must be administered to the patient freshly prepared, and where this is not possible [99mTc]gluconate has been used instead. The latter has the relative disadvantage of lower uptake by the kidneys. 7 The radiopharmaceuticals used for renography, [99~Tc]diethylenetriaminepentaacetic acid (DTPA), [99mTC] mercaptoacetyltriglycine (MAG3), and iodine123 (1231) hippuran undergo rapid changes in uptake and distribution in the kidney and are therefore considered unsuitable for SPECT. Any change in the object during SPECT data acquisition causes artifacts in the reconstructed images. Changes in uptake of the renography agents take place in times that are very short in comparison to the data acquisition period necessary for satisfactory images with a rotating gamma camera. Imaging of the kidneys by SPECT is technically feasible using other agents in which uptake in the kidney is incidental to their primary clinical use. Examples include the use of bone imaging agents such as [99mTc]methylene diphosphonate (MDP) and thallium-201 (2~ Agents
114
E. DAVID WILLIAMS
used to seek infection, such as gallium-67 (67Ga) citrate or indium-Ill-labeled white blood cells may also be localized in or near the kidneys. The most suitable radiopharmaceutical is undoubtedly [99mTc]DMSA, which gives a very high ratio of target organ uptake to background. Uptake is also constant during data collection at the usual interval after administration of about 3 hours or more. WHEN TO DO SPECT: CLINICAL APPLICATIONS
Renal Masses
The detection of space-occupying lesions in the kidney, suspected on the basis of intrave-
nous urography, was an early area of investigation. 8-13The clinical utility of SPECT in comparison to conventional gamma camera imaging has been assessed in a study of 60 patients suspected of having space-occupying lesions of the kidney) 4 A rotating gamma camera and [99~Tc]DMSA were used (Fig 1). In several individual cases (Figs 2 and 3), tomography had aided diagnosis, for example in an obese patient where an alternative noninvasive investigation (ultrasound) was inconclusive. However, objective trials using four observers showed only a slight, statistically insignificant advantage from the addition of tomographic to conventional images (Fig 4). Not surprisingly, as the size of
RP0
LPO
D
R
L Fig 1. SPECT in a normal subject. (A) normal planar views; (8) transverse, (C) sagittal, (D) coronal tomographic sections, Note that following convention, planar scans are shown as posterior views, but coronal sections are shown as anterior views. Sagittal sections are viewed from the right lateral position with the patient's anterior on the right. Transverse sections are viewed from the patient's feet with anterior at the top of the picture. P, posterior; L, left; R, right; RPO, right posterior oblique; LPO, left posterior oblique. (Reprinted with permission)')
RENAL SPECT
115
|PO
Fig 2. Renal tumor (clear cell renal adenocarcinoma at nephrectomy). (A) Planar views show only a slight abnormality in the lower pole of the right kidney. (B) Sagittal section shows a clear defect in the lower pole of right kidney. (Reprinted with permission. 14)
the lesion increased, detection and localization were improved. The addition of tomograms consistently gave better results (Fig 5). However, there is also a need to characterize such lesions, distinguishing between benign cysts and renal tumors. These aims have generally been better achieved by ultrasound and CT scanning. Thus, SPECT has limited potential in this area. Ultrasound now seems to be the more usual initial imaging investigation, followed by CT in cases where ultrasound has been inconclusive
or inadequate. Following an earlier investigation 15 that showed that in some tumors increased vascularity could be seen on planar imaging, Wujanto et a116reported initial promising results of the use of blood pool SPECT imaging in association with renal SPECT to characterize kidney masses. Unfortunately, failure to find a vascular lesion does not exclude a tumor. 17 A renal pseudotumor (or column of Bertin) can be a source of confusion in radiographic
bO A
6 B
Fig 3. Renal tumor. (A) Planar views show slightly reduced uptake in the lower pole of the right kidney. (B) Coronal tomographic sections show a definite defect. The ultrasound scan was of poor quality owing to the patient's obesity, and therefore no lesions could be identified. A solid tumor was demonstrated by angiography and confirmed at surgery. (Reprinted with permission.14)
116
E. DAVID WILLIAMS
PERCENTFALSEPOSITIVE 0
PERCENT
lO0 0
o
lO0 0
t
I
Ol
Observer2
Observer 3
Ob. . . . . . l
1
TRUE
.;~)~-9_-=- "
POSITIVE
o planar scans
l
Ot
lO0
Observer4
I
9 Pooled
with tomegrams
responses
Fig 4. Receiver operating characteristic (ROC) curves for four observers for detection of renal lesions on planar images, and when presented with tomograms. Results for the pooled responses o f all observers are also shown. There was no significant difference between the areas under the ROC curves obtained with and without tomograms. (Reprinted w i t h pennission? 4)
investigation of the kidney because it can simulate a renal tumor on an excretion urogram. However, the ectopic cortical tissue takes up [99~Tc]DMSA. While on planar imaging nothing abnormal may be seen, on SPECT imaging this ectopic uptake may be clearly seen ]4,1820(Fig 6). The advantage in using SPECT here is that it enables uptake in the center of the kidney to be demonstrated, which is hidden by overlying cortical uptake on planar images. Another instance in which SPECT has been used to demonstrate ectopic renal tissue is in the case of horseshoe kidney. Planar imaging does not usually show tissue linking the two kidneys anterior to the spine clearly. In such cases, SPECT can confirm what may have been suspected from the initial planar images. 2L22 l O0
PERCENT
80
TRUE POSITIVE 60
with \ tomogram
LOCATION OF LESION 40
planar ~m
0 0 - 2
2 - 4 LESION SIZE
4 - 6
over 6
(cm)
Fig 5. Percentage of true-positive scans for detection and localization was higher for larger lesions and slightly higher w h e n tomograms were added. (Reprinted w i t h permission. '4)
SPECT has also been used to elucidate initially confusing images obtained for localization of 67Ga in a perinephric abscess23 and in an infected cyst in a polycystic kidney. 24The advantage conferred by SPECT seems to have been in enabling uptake in adjacent tissues, giving overlying planar images, to be separated. Technetium-99m DMSA SPECT has been used to demonstrate two other types of lesion not clearly shown on planar scans, renal infarct and hematoma, z~
Individual Kidney Function Renography is the most commonly used method for assessing individual kidney function. An "absolute" measure might be obtained by measuring inulin clearance and, separately, relative renal function with a radiopharmaceutical. However, measurement of the percentage uptake of injected radioactivity of [99mTc]DMSA in the kidney has also been suggested as a measure of individual kidney function. 26Techniques that have been described include the use of singleview (usually posterior) planar imaging, with estimation of renal depth, and calculation of uptake by comparison with a phantom. The use of both anterior and posterior planar images is generally more satisfactory and more convenient than measuring the depth of the kidneys. 27 SPECT imaging could be the best methodY having potentially more accurate attenuation corrections, ~ and the possibility of avoiding interference from radioactivity in the adjacent liver. Studies comparing subjects with one or two kidneys showed uptake of 20% of injected activity in each of two kidneys, and 35% in a solitary kidney (after nephrectomy) (Fig 7). There was less uptake in diseased kidneys) 9 Further studies showed a good correlation between percentage uptake and creatinine clearance in subjects with a single kidney. 3~ Thus, there seems to be a good relationship between percentage uptake and the function of each kidney. One reservation in measuring renal function this way is that an impure radiopharmaceutical, resulting from too long a time between preparation and use, will give relatively reduced renal uptake.
Renal Volume SPECT has been used to estimate functioning organ volume, 31 including that of the kidney. 32
RENAL SPECT
117
Fig 6. Renal pseudotumor. Urography showed compression of the calyces of the right kidney, indicating a possible renal mass. Ultrasound showed solid tissue in the middle of the right kidney and a similar but smaller area in the left kidney. (A) Conventional views are normal. (B) Sagittal tomograms show uptake in the middle of each kidney in columns of Bertin. (Reprinted with permission. 14)
Wujanto et a133 investigated the use of the method as a means of classifying renal scarring9 They found that there was wide variation in the measured volumes of both scarred and unscarred kidneys. However, they suggested that
Relative Renal Function
60
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EgJ
~l~'rl~
,-~
40
L~T
eI~TE~V.
RIGHT
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anatomical abnormalities, and for quantitative studies of renal uptake and volume. In these areas, it has provided little advantage over conventional imaging, but it has been helpful in individual cases. Highresolution SPECT is a promising new development, which may have applications in detecting and classifying renal scarring. It deserves careful evaluation.
of single photon emisT HEsionTECHNIQUE computed tomography (SPECT) is
distribution of a radiopharmaceutical in the body. This is therefore the route both to better image contrast and hence quality, and to quantify radioactivity distributions. 1 There has been relatively little use of SPECT for dynamic studies, mainly because after administration of the amounts of radioactivity currently considered acceptable for diagnostic use, acquisition of sufficient counts to reconstruct useful images takes about 10 minutes with a single rotating gamma camera. 2 Thus, only very slow dynamic studies are possible. Newer, high-sensitivity equipment may permit dynamic tomographic studies, but no such studies of the kidney have been reported yet. This review is therefore concerned only with "static" imaging, ie, when the amount and location of radioactivity does not change during image data acquisition. Many static imaging investigations in clinical nuclear medicine are concerned with detection of lesions, rather than estimation of whole organ function. Experience to date suggests that SPECT may provide improvements in detection of lesions when looking for defects in a region of fairly uniform uptake, or where the normal pattern of uptake in the region of the lesion is complex. As an example of the latter situation, while planar imaging appears adequate for much of bone scintigraphy, SPECT has been found useful in the skull, lumbar spine, and hip joints. Earlier in the history of SPECT, liver imaging seemed to be a promising area of the former type of application, but more recently radionuclide imaging for detection of lesions in the liver has largely been superceded by ultrasound and computed tomography (CT), which can generally detect smaller lesions and indicate their nature. The principal diagnostic appli-
now well established in clinical nuclear medicine for myocardial and cerebral perfusion imaging. It is indeed the standard imaging technique for these applications in many departments. While SPECT is also used for imaging other organs, such applications tend to be restricted to second-line or specialized investigations. This review concerns the question of whether SPECT can and should be used for renal investigations. This question is primarily concerned with the technical advantages that SPECT may (or may not) have over planar imaging. Three components to the question need to be answered: why should it be used, how should it be applied, and when--to what clinical problems? WHY SPECT? THE PHYSICAL SITUATION
Planar imaging with a gamma camera is the "normal" method used throughout clinical nuclear medicine. It is relatively simple to use, in that for static imaging no processing beyond the display of images is required. It is also readily applicable to dynamic studies. In contrast, SPECT requires more specialized equipment and generally takes longer to acquire, process, and display images. Its use therefore requires special justification. However, emission computed tomography is now seen as the method that can provide more accurate depiction of the
From the Regional Medical Physics Department, District General Hospital, Sunderland, UI~ Address reprint requests to E. David 14711lares, PhD, Regional Medical Physics Department, District General Hospital, Sunderland SR4 7TP, UIC Copyright 9 1992 by W..B. Saunders Company 0001-2998/92/2202-0005505.00/0 112
Copyright9
1992 b y W.B. Saunders
Company
Seminars in Nuclear Medicine, Vol XXll, No 2 (April), 1992: pp 112-121
RENAL SPECT
113
cations of SPECT in recent years have been for imaging perfusion in the brain and myocardium. A common factor in both of these applications is that defects of uptake are sought in a situation where there is fairly uniform uptake throughout the organ, and defects may be obscured by uptake in adjacent tissues. This situation, of a "cold target," is one where improving image contrast can give better lesion detection. A conclusion that may be extrapolated from this is that since the kidney presents a physically similar problem--the renal parenchyma has morphological similarity to the myocardium and lesions also give decreased uptake--it is therefore a potentially suitable organ for application of SPECT with its good image contrast. In addition, whole kidney function is also, and more usually, of clinical interest, and therefore so are methods for improving assessment of function. HOW
lesion contrast and minimizing image noise. Commercial software and terminology may vary so much that specific recommendations are difficult. Therefore, use of a phantom that simulates as closely as possible the size of the patient, the count rate, and the physical detail to be demonstrated is highly recommended. General purpose body section phantoms may well be adequate, although they do not usually provide objects similar to the kidneys. A range of reconstruction filters and display parameters should be tried and the optimum then selected for clinical use. A compromise must be found between factors that give the best contrast, such as a narrow slice width and "sharp" spatial filter, and the opposite (wide slice width, "smooth" filter) which give a smooth, low-noise image. Parasagittal and coronal sections are often much more useful than transverse sections: display of all three planes is therefore recommended. Oblique sections may also be helpful.
Equipment and Techniques The basic principles of SPECT, in which gamma emissions from the subject are gathered over a wide angle and used for a computer reconstruction of section images of the object, are well described in monographs on the subject 3and in textbooks dealing with the physics of nuclear medicine 4 and so will not be repeated here. A rotating gamma camera system in which a standard gamma camera detector is rotated about a horizontal axis through the patient who lies on a narrow couch, has been the most widely used system for data acquisition over the last 10 years. A new, and perhaps highly significant, development has been the introduction of triple gamma camera SPECT systems, which provide a substantial increase in sensitivity. 5 Such systems encourage the use of higher resolution collimators, which are an essential component in attempting detection of smaller lesions. 6 In any application of SPECT, attention to details of technique cannot be overemphasized. Quality control of the equipment, including regular assessment of and correction for gamma camera nonuniformity, is essential, as are mechanical stability of the gantry and correction for center of rotation offset. Appropriate slice thickness, filtering, and display parameters must also be selected with the aim of maximizing
Radiopharmaceuticals SPECT has been applied to renal imaging using static agents only. Technetium-99m (99mTc) dimercaptosuccinate (DMSA) has been most widely used. However, it must be administered to the patient freshly prepared, and where this is not possible [99mTc]gluconate has been used instead. The latter has the relative disadvantage of lower uptake by the kidneys. 7 The radiopharmaceuticals used for renography, [99~Tc]diethylenetriaminepentaacetic acid (DTPA), [99mTC] mercaptoacetyltriglycine (MAG3), and iodine123 (1231) hippuran undergo rapid changes in uptake and distribution in the kidney and are therefore considered unsuitable for SPECT. Any change in the object during SPECT data acquisition causes artifacts in the reconstructed images. Changes in uptake of the renography agents take place in times that are very short in comparison to the data acquisition period necessary for satisfactory images with a rotating gamma camera. Imaging of the kidneys by SPECT is technically feasible using other agents in which uptake in the kidney is incidental to their primary clinical use. Examples include the use of bone imaging agents such as [99mTc]methylene diphosphonate (MDP) and thallium-201 (2~ Agents
114
E. DAVID WILLIAMS
used to seek infection, such as gallium-67 (67Ga) citrate or indium-Ill-labeled white blood cells may also be localized in or near the kidneys. The most suitable radiopharmaceutical is undoubtedly [99mTc]DMSA, which gives a very high ratio of target organ uptake to background. Uptake is also constant during data collection at the usual interval after administration of about 3 hours or more. WHEN TO DO SPECT: CLINICAL APPLICATIONS
Renal Masses
The detection of space-occupying lesions in the kidney, suspected on the basis of intrave-
nous urography, was an early area of investigation. 8-13The clinical utility of SPECT in comparison to conventional gamma camera imaging has been assessed in a study of 60 patients suspected of having space-occupying lesions of the kidney) 4 A rotating gamma camera and [99~Tc]DMSA were used (Fig 1). In several individual cases (Figs 2 and 3), tomography had aided diagnosis, for example in an obese patient where an alternative noninvasive investigation (ultrasound) was inconclusive. However, objective trials using four observers showed only a slight, statistically insignificant advantage from the addition of tomographic to conventional images (Fig 4). Not surprisingly, as the size of
RP0
LPO
D
R
L Fig 1. SPECT in a normal subject. (A) normal planar views; (8) transverse, (C) sagittal, (D) coronal tomographic sections, Note that following convention, planar scans are shown as posterior views, but coronal sections are shown as anterior views. Sagittal sections are viewed from the right lateral position with the patient's anterior on the right. Transverse sections are viewed from the patient's feet with anterior at the top of the picture. P, posterior; L, left; R, right; RPO, right posterior oblique; LPO, left posterior oblique. (Reprinted with permission)')
RENAL SPECT
115
|PO
Fig 2. Renal tumor (clear cell renal adenocarcinoma at nephrectomy). (A) Planar views show only a slight abnormality in the lower pole of the right kidney. (B) Sagittal section shows a clear defect in the lower pole of right kidney. (Reprinted with permission. 14)
the lesion increased, detection and localization were improved. The addition of tomograms consistently gave better results (Fig 5). However, there is also a need to characterize such lesions, distinguishing between benign cysts and renal tumors. These aims have generally been better achieved by ultrasound and CT scanning. Thus, SPECT has limited potential in this area. Ultrasound now seems to be the more usual initial imaging investigation, followed by CT in cases where ultrasound has been inconclusive
or inadequate. Following an earlier investigation 15 that showed that in some tumors increased vascularity could be seen on planar imaging, Wujanto et a116reported initial promising results of the use of blood pool SPECT imaging in association with renal SPECT to characterize kidney masses. Unfortunately, failure to find a vascular lesion does not exclude a tumor. 17 A renal pseudotumor (or column of Bertin) can be a source of confusion in radiographic
bO A
6 B
Fig 3. Renal tumor. (A) Planar views show slightly reduced uptake in the lower pole of the right kidney. (B) Coronal tomographic sections show a definite defect. The ultrasound scan was of poor quality owing to the patient's obesity, and therefore no lesions could be identified. A solid tumor was demonstrated by angiography and confirmed at surgery. (Reprinted with permission.14)
116
E. DAVID WILLIAMS
PERCENTFALSEPOSITIVE 0
PERCENT
lO0 0
o
lO0 0
t
I
Ol
Observer2
Observer 3
Ob. . . . . . l
1
TRUE
.;~)~-9_-=- "
POSITIVE
o planar scans
l
Ot
lO0
Observer4
I
9 Pooled
with tomegrams
responses
Fig 4. Receiver operating characteristic (ROC) curves for four observers for detection of renal lesions on planar images, and when presented with tomograms. Results for the pooled responses o f all observers are also shown. There was no significant difference between the areas under the ROC curves obtained with and without tomograms. (Reprinted w i t h pennission? 4)
investigation of the kidney because it can simulate a renal tumor on an excretion urogram. However, the ectopic cortical tissue takes up [99~Tc]DMSA. While on planar imaging nothing abnormal may be seen, on SPECT imaging this ectopic uptake may be clearly seen ]4,1820(Fig 6). The advantage in using SPECT here is that it enables uptake in the center of the kidney to be demonstrated, which is hidden by overlying cortical uptake on planar images. Another instance in which SPECT has been used to demonstrate ectopic renal tissue is in the case of horseshoe kidney. Planar imaging does not usually show tissue linking the two kidneys anterior to the spine clearly. In such cases, SPECT can confirm what may have been suspected from the initial planar images. 2L22 l O0
PERCENT
80
TRUE POSITIVE 60
with \ tomogram
LOCATION OF LESION 40
planar ~m
0 0 - 2
2 - 4 LESION SIZE
4 - 6
over 6
(cm)
Fig 5. Percentage of true-positive scans for detection and localization was higher for larger lesions and slightly higher w h e n tomograms were added. (Reprinted w i t h permission. '4)
SPECT has also been used to elucidate initially confusing images obtained for localization of 67Ga in a perinephric abscess23 and in an infected cyst in a polycystic kidney. 24The advantage conferred by SPECT seems to have been in enabling uptake in adjacent tissues, giving overlying planar images, to be separated. Technetium-99m DMSA SPECT has been used to demonstrate two other types of lesion not clearly shown on planar scans, renal infarct and hematoma, z~
Individual Kidney Function Renography is the most commonly used method for assessing individual kidney function. An "absolute" measure might be obtained by measuring inulin clearance and, separately, relative renal function with a radiopharmaceutical. However, measurement of the percentage uptake of injected radioactivity of [99mTc]DMSA in the kidney has also been suggested as a measure of individual kidney function. 26Techniques that have been described include the use of singleview (usually posterior) planar imaging, with estimation of renal depth, and calculation of uptake by comparison with a phantom. The use of both anterior and posterior planar images is generally more satisfactory and more convenient than measuring the depth of the kidneys. 27 SPECT imaging could be the best methodY having potentially more accurate attenuation corrections, ~ and the possibility of avoiding interference from radioactivity in the adjacent liver. Studies comparing subjects with one or two kidneys showed uptake of 20% of injected activity in each of two kidneys, and 35% in a solitary kidney (after nephrectomy) (Fig 7). There was less uptake in diseased kidneys) 9 Further studies showed a good correlation between percentage uptake and creatinine clearance in subjects with a single kidney. 3~ Thus, there seems to be a good relationship between percentage uptake and the function of each kidney. One reservation in measuring renal function this way is that an impure radiopharmaceutical, resulting from too long a time between preparation and use, will give relatively reduced renal uptake.
Renal Volume SPECT has been used to estimate functioning organ volume, 31 including that of the kidney. 32
RENAL SPECT
117
Fig 6. Renal pseudotumor. Urography showed compression of the calyces of the right kidney, indicating a possible renal mass. Ultrasound showed solid tissue in the middle of the right kidney and a similar but smaller area in the left kidney. (A) Conventional views are normal. (B) Sagittal tomograms show uptake in the middle of each kidney in columns of Bertin. (Reprinted with permission. 14)
Wujanto et a133 investigated the use of the method as a means of classifying renal scarring9 They found that there was wide variation in the measured volumes of both scarred and unscarred kidneys. However, they suggested that
Relative Renal Function
60
so.. KIDI~-Y
EgJ
~l~'rl~
,-~
40
L~T
eI~TE~V.
RIGHT
B
