Dushyant Sahani, MD

Computer-aided detection is gradually gaining acceptance in radiology and has become a major research focus in the past few years. The development of CAD with multislice CT has reached the point where, together, they have the potential to offer new capabilities in the interpretation of emergency room scans.

Various versions of the CAD acronym have been suggested, including computer-aided detection, computer-aided diagnosis, and computer-assisted detection. Today, it is universally accepted that any CAD applications will assist a radiologist rather than serve as independent diagnostic tools, and therefore the term "detection" is more appropriate.

 The recognition of virtual colonoscopy for the screening of precancerous colonic polyps and of CAD for generating automated segmentations of liver and kidney has encouraged further work to auto-mate advanced postprocessing applications.

 Although the potential of computer-aided techniques for use with MRI data sets has been explored for a few clinical needs, there is more acceptance of CAD algorithms with MSCT.

Novel methodologies and techniques, like CAD, should always be directed to look for solutions to common and routine problems. Emergency radiology accounts for 30% to 40% of all CT scans and is important to patient care because of the frequency of life-threatening presentations in the emergency room. Given the importance of emergency radiology and its potentially crucial impact on standards of patient care, it would be wise to encourage the development of MSCT-CAD applications geared to solving the common problems in diagnosis of acute, life-threatening clinical conditions.

Certain advantages of MSCT are unique to an emergency radiology setting. Generally, a seriously ill or injured patient is likely to be imaged on a modality that ensures good accuracy and reinforces diagnostic confidence of radiologists and clinicians. MSCT has performed the best so far in maintaining the delicate balance between accuracy, readers' confidence, and time savings in the emergency room. MSCT also offers the best diagnostic accuracy for most clinical conditions that present in emergency situations and is preferred over MRI because of its faster scan speed and better image resolution. These qualities also lower the probability of encountering motion artifacts in the images, which is a common problem in an emergency radiology setting.

Research and development that explore the utility of computer-aided techniques has been uneven across the various dimensions of clinical practice. Practitioners in a clinical setting put up with a high rate of false-positives because of the better accuracy and specificity of CAD results. But concerns over false-positive results have delayed exploration of CAD's potential in an emergency radiology setting, where the instant and accurate detection of life-threatening conditions is of paramount importance.

The need for scan data sets conducive to postprocessing and the difficulty of integrating CAD results from a 3D workstation into PACS form the basis for most apprehension. However, the advent of MSCT and the ever-improving resolution of CT data sets have opened the door to development of pulmonary embolus detectors and ways to locate intracranial bleeding sites.

The ability to maximize a lesion's conspicuity and achieve a clear definition of its borders is essential to ensure acceptable accuracy in CAD results. A computer-aided lesion detector can produce optimal results only if the lesion borders are well defined. Fuzziness around the lesion boundaries of lung nodules was routinely encountered with earlier generation CT scanners when sections were obtained at thicknesses of more than 5 mm. This increased the number of false positives not only through erroneous mapping of adjacent vasculature but also by appearing to alter the actual size of lesions. The availability of thin sections with 16-slice and higher scanners has addressed this issue and led to potential solutions.

Image resolution further influences the clarity of lesion borders. The faster scan speeds available on newer generation CT scanners have dramatically decreased the probability of mo-tion artifacts, thus providing a solu-tion to another problem of the ER setting.

The CAD highlights for emergency radiology that have been displayed in various technical journals and conference platforms have used sophisticated technical terms like level-set and thresholding to describe CAD techniques. The crucial initial step in development of any computer-aided CT lesion detector in most situations is defining a preset range of Hounsfield units such that CAD can display the voxels that fall in the assigned range; e.g., −900 to −930 HU for detection of free air in cases of pneumothorax.

Voxels, which are mapped in color by CAD, are then used to further facilitate depiction of lesion volume and total lesion burden. The same considerations are also the basis for extending CAD concepts beyond lesion detection to facilitate automation in postprocessing.

Another basic principle that is being used to further drive development of CAD applications is the detection of pathologies based on variations in lumen diameter. CT colonoscopy has provided an ideal platform for virtual-navigation CAD, and the same concepts are being applied for detection of transition points of small bowel obstructions.

In terms of research, the exploration of CAD applications in ER is relatively new. A realization of the potential importance of computer-aided principles in emergency radiology is too recent for much data to have been disseminated broadly. Its popularity has been restricted to various conference platforms so far, and data may be waiting in the pipeline for publication.

CAD's potential for detecting intracranial bleeding sites and pulmonary embolism has been established by studies recently published in various journals. However, an interesting array    of emergencies in which the new role of CAD is gaining momentum deserves special    mention.

• Pneumothorax. The importance of automated detection and quantification of pneu-mothorax received prominent recognition at the 2007 RSNA meeting.

 Assigning CAD to use a range of Hounsfield units on MSCT data sets for mapping free air in the pleura of trauma patients revealed promising accuracy in quantification of pneumothorax and also highlighted the time savings. With computer-aided automated volume-try finding a strong correlation coefficient-nearly 0.9-between the clinical score and manually measured free air in pleural space, promising results are eagerly anticipated.

Results already achieved have encouraged researchers to go a step further and test CAD performance for quantifying pneumothorax in emphysematous patients and work on erroneous mapping of emphysematous bullae present in the lung. Clinicians wishing to ensure clinical stability may continue to place a chest tube.

This may sound discouraging for those attempting to develop CAD's ability to quantify free air in the pleura, but CAD can still play a role in the emergency room for monitoring of a relatively stable patient with a small pneumothorax. Other potential roles include situations just beginning to be explored, such as post-lung biopsy. Here, the idea is to avoid an invasive procedure in an otherwise stable patient.

• Pulmonary embolism. PE is a life-threatening emergency requiring prompt diagnosis to ensure a good clinical outcome. It is the only condition routinely encountered in the emergency setting for which the use of CAD has been extensively investigated.

 Newer computer-aided detectors have the ability to automatically map tiny clots, even in the sixth-order branches of pulmonary vasculature. With the recent advances in scanners and scanning techniques, sensitivity has been as high as 85%. Specificity has ranged between 90% and 95%.

The technical learning curve with the improvement of scan resolution and speed has brought down the false positive per patient rate from 4.5 to two. More recent studies have high-lighted the benefits of adding CT venography to regular PE protocols on MSCT, which raises sensitivity to 90%. Some clinicians may still prefer going through the scan data slice by slice, but it seems possible that CAD could be used as a second opinion for conditions such as PE in which care and vigilance in reporting blood clots is important.

• Small bowel obstruction. Use of CAD for locating transition points of small bowel obstructions, such as bowel strictures, was investigated in a 15-patient study by Sainani et al11 that found that CAD identified at least 60% of surgically confirmed strictures with a false positive rate of three per patient.

 Of these, 80% were easily dismissed as erroneous CAD points in vessels, abdominal wall, or collapsed large bowel segments. Considering that redundancy and variability in small bowel position are likely to continue posing challenges, even small cohort pilot studies such as this may point to a solution.

Exploration of CAD's potential in the emergency radiology setting is just beginning. At this vital point of technical advancement, it may be time to focus on CAD research. With the use of terms such as "detection" instead of "diagnosis," a broader meaning that includes automated postprocessing techniques can be included under the definition of CAD.

Patient care in the emergency setting continues to be a challenge. Patients are frequently scanned with CT, and directing CAD research to find ways to quickly and accurately diagnose life-threatening emergencies could have immediate and far-reaching benefits. Improving CAD of thoracic and abdominal problems, which are responsible for a considerable number of emergency presentations, may help reduce mortality and morbidity in the emergency setting.

One can imagine that the CAD concept of shape-based detection, which is used for colonic polyps, may also work for detection of tiny causes of abdominal pain such as a renal stone that may be overlooked by the naked eye. However, false-positive detections due to renal artery calcification and inclusion of aortic calcification foci within the region of interest may pose challenges for improving software accuracy.

CAD colonoscopy that can detect polyps as small as 5 mm in diameter has already paved a path for further application of such concepts. This may encourage attempts to use CAD to locate occult bleeding sites in the small bowel lumen and, if that's successful, may also be used in conjunction with digital subtraction images toimprove accuracy in mapping the bleeding sites.

At this point, CAD's success in locating such occult bleeds may be variable, considering the sites' tiny size, possibly even smaller than 5 mm. But, thanks to such advances in scanners as high-resolution CT, more refined MSCT data may soon be available, which may make generation of such applications possible.

• Musculoskeletal emergencies. It's also possible to imagine developing CAD fracture locators in patients who present with multitrauma. Such an application might have a role in detecting hairline fractures not visible to the naked eye on coronal and sagittal reformats.

A possible danger in this approach is that such CAD algorithms may be programmed to detect subtle discontinuities along the bone's cortical surface that may be confused with artifacts, thus giving rise to false-positive detections. Ensuring exclusion of joints in mapping CAD results may decrease the number of false positives to some extent, but the condition of patients in the emergency setting and the possibility of motion artifacts may pose hurdles and slow research.

• A CAD-conducive research environment. It is important to ensure a good initial groundwork by considering the viewpoints of various schools of thought before formulating a future CAD project. Investigating CAD algorithms on a prospective basis in an emergency setting may provide more reliable answers to queries and misconceptions about CAD and thus help in tapping its future potential.

Setting up a close interaction between an institution's 3D laboratory and the emergency room may provide vital information on some of the common clinical emergencies while we await technical advances in scanner consoles and the integration of CAD and PACS.

The emergency room is a setting that demands vigilance to ensure good patient care and contributes a considerable volume of CT to the radiology workflow. Understanding its importance, and considering the advances that newer MSCT scanners can offer, it would be prudent to add more weight to CAD research in emergency radiology.

With emerging answers for spotting intracranial bleeding locations, tiny pulmonary emboli, pneumothorax, and transition sites of small bowel obstructions, we can hope for a more aggressive approach by CAD researchers to providing diagnostic solutions to emergencies while simultaneously refining CAD's capability to act as a "second reader." Research and development of CAD concepts in busy emergency scanner console and reporting rooms should be promoted to explore the parameters and dimensions of CAD in patient care.

Dr. Singh is a radiology research fellow, and Dr. Yoshida is head of the research group, both in the 3D imaging department at Massachusetts General Hospital in Boston. Dr. Sahani is director of CT at MGH and an associate professor of radiology at Harvard University.

Articles

Multislice CT and CAD bring new efficiencies to diagnosis

The most crucial process to assess in a tumor for efficient management is neo-angiogenesis. Currently, no reliable biomarker is available to measure angiogenesis, and direct microvessel density assay requires frequent biopsies for serial tumor monitoring. Technological innovations in imaging, however, have enabled the capture of physiological information in addition to anatomic details. One promising imaging technique for acquiring physiological information about angiogenesis is perfusion CT,

 which provides anatomic information as well.

Initial reports on dynamic perfusion CT date back to 1980,

 But the limitations of earlier CT scanners and a lack of availability of analytic software restricted this technique to use as a research tool. Recent developments in multislice CT and new availability of commercial perfusion software, however, have led to a resurgence of clinical interest in perfusion CT, with rapid expansion of its applications. The clinical role of perfusion CT in stroke management is well established, but its role in oncology for diagnosis, risk stratification, and therapeutic response monitoring continues to evolve.

Clinical management in oncology shifted after the concept of tumor neo-angiogenesis was introduced. This process of new blood vessel growth is critical for tumor growth and metastasis.

 Newer generations of oncologic treatment regimens target neo-angiogenesis, delivering less systemic toxicity than did conventional cytotoxic agents. Anti-angiogenic drugs are targeted specifically to the growth factors that stimulate neo-angiogenesis.

In tumor tissue, the release of several abnormal mediators such as vascular endothelial growth factor (VEGF)

 leads to development of a new network of capillaries. Angiogenic vasculature differs in several aspects from that in normal mature tissue. Blood vessels in tumors are typically dilated and tortuous, with abnormal branching patterns and dead ends, and they lack organized structure into arterioles, capillaries, and venules.

 These tumor blood vessels have increased permeability over normal tissue.

Angiogenic factors increase microvessel density, which is defined as the mean number of endothelial cells or endothelial cell clusters in a given number of microscopic fields that are clearly separate from adjacent tumor cells and connective tissue elements.

 Microvessels are highlighted by staining the endothelial cells with anti-CD34 antibodies.

Microvessel density has been established as a prognostic indicator for many cancers, and the most direct strategy to monitor anti-angiogenic therapy would therefore be periodic biopsy. This approach is invasive, however, and likely unacceptable to patients. Moreover, estimates of microvessel density may not reflect the overall tumor biology if a single-site biopsy is used, due to the heterogeneity of perfusion within the tumor.

Noninvasive imaging, on the other hand, can be repeated in patients with larger volume coverage than biopsies. Imaging studies allow assessment of physiological parameters-such as blood volume, blood flow, permeability, and mean transit times-that reflect tumor biology. Imaging may therefore be superior to microvessel density measurement for assessing angiogenesis.

 Perfusion CT involves dynamic scanning after administration of iodinated contrast material followed by mathematic modeling to study contrast material kinetics in the tissue. Estimates of tumor perfusion parameters such as blood flow, blood volume, mean transit time, and permeability-surface area product can be derived by applying kinetic models to the CT perfusion data set (see table).

Among the various mathematic models that have evolved for performing perfusion studies, the deconvolution method is currently the most validated and widely used approach. Perfusion CT involves acquisition of unenhanced images followed by a series of postcontrast dynamic images in axial cine mode with static table position. The unenhanced images are obtained to select the region of interest for dynamic imaging of tumor perfusion. Because current CT techniques (four- to 64-slice CT) allow scanning of only 2 to 4 cm of tumor tissue, careful selection of region of interest is key. Breath-hold is crucial for perfusion CT study of upper abdominal viscera where breathing artifacts are more likely. Duration of scanning ranges from 30 seconds to two minutes from arrival of contrast, depending on clinical need, organ of interest, and analytical method.

Time of arrival of contrast may vary depending on the location of the tumor. Contrast would arrive earlier in a lung mass, for example, than a mass in the foot. Earlier reports recommended scanning for at least two minutes when permeability measurement was required,

 but current recommendations advocate shorter durations. A recent study reported that a 65-second scan is adequate for measurement of permeability of colorectal carcinoma.

 Radiation dose, breath-hold demands, and patient motion invariably limit the maximum duration of scanning. There is always a trade-off between radiation dose and temporal resolution.

Clinical imaging tools for evaluation of angiogenesis should help the oncologist select optimum treatment by tailoring the dose of anti-angiogenic drugs, detecting early anti-angiogenic response, and analyzing impact of treatment on tumor progression.

Data on body tumor imaging with perfusion CT are emerging in studies of various applications. One drawback is the wide variability of scanning techniques and mathematical modeling used in these studies. Furthermore, most of the studies use a small sample. Readers should be aware of these limitations before drawing conclusions from these studies and implementing these applications in routine practice.

 Liver tumors. The liver is a unique organ for perfusion CT imaging because it has two arterial inputs: the hepatic artery and the portal vein. Malignant liver lesions such as hepatocellular carcinoma have predominantly a hepatic arterial supply and thus demonstrate increased arterial tumor perfusion values in CT scans. Studies have established that micrometastases alter liver perfusion despite the absence of gross disease.

 Hence, it is conceivable that this technique might help detection of micrometastases in selected patients by mapping areas of altered liver perfusion.

It has been shown that in healthy individuals, CT-derived hepatic arterial and portal venous perfusion values are inversely correlated, whereas in a setting of liver metastases, there is a direct positive correlation between these values.

 Miles et al reported that increased peritumoral arterial perfusion and increased global liver perfusion indicate a more favorable outcome following chemotherapy.

 Tsushima et al reported that HCC has nine times greater hepatic arterial perfusion than background liver parenchyma, whereas colorectal carcinoma metastases demonstrate only four times greater arterial perfusion.

 A few patients with HCC showed a decrease in hepatic perfusion values following transarterial chemoembolization.

 Successful surgical management of pancreatic adenocarcinoma is often fraught with difficulties due to late clinical presentation and the limited ability of cross-sectional imaging modalities to detect small lesions. Sensitivity in detecting a pancreatic mass 2 cm or smaller is 58% to 77%, even with the use of MSCT.

 Pancreatic adenocarcinomas are usually hypovascular, however, and detection of change in this perfusion pattern could aid in early detection, when these masses are still resectable. Other tumors and disease processes in the pancreas may also alter pancreatic perfusion. 

Miles et al reported a decrease in pancreatic perfusion in patients with type 1 diabetes mellitus and failing pancreatic transplant, whereas pancreatic perfusion increased in insulinoma and Wilson's disease compared with healthy individuals.

 Another study reported increased blood flow and blood volume in insulinoma compared with background pancreatic parenchyma. Time to peak enhancement and tumor permeability were comparable to normal pancreas, however.

 Even in the absence of disease in the pancreas, it has been reported that pancreatic perfusion decreases with advancing age.

 Abe et al validated pancreatic perfusion CT by showing excellent agreement of pancreatic tumor blood flow measurement between perfusion CT and xenon CT.

. CT perfusion is evolving as a promising tool for evaluation of rectal carcinoma and treatment monitoring. Goh et al investigated the optimum scanning duration and z-axis coverage for perfusion CT of colorectal cancer.

 They reported that a 65-second scanning duration is required for reliable permeability measurements, and increasing the z-axis coverage did not improve the reproducibility of perfusion measurements.

 In our institution, we have investigated the role of perfusion CT before and after treatment in patients with rectal carcinoma to monitor treatment response. We found that rectal carcinoma showed substantially higher blood flow (Figure 1) and shorter mean transit time when compared with normal rectal wall.

 Following chemoradiation treatment, rectal carcinoma showed considerable reduction in blood flow and increase in mean transit time.

 Early anti-angiogenic treatment effects can also be monitored with this technique. Willet et al reported a drop in tumor perfusion in patients with rectal carcinoma two weeks after initiation of bevacizumab therapy, further supporting the role of perfusion CT in tumor angiogenesis assessment.

. Tumor size and the location of the prostate gland often make detection of prostatic carcinoma by imaging challenging. Although tumor visualization is improved using transrectal ultrasound and MRI, perfusion CT enables direct visualization of tumor physiology and aids treatment planning (Figure 2). Prior studies reporting that cancer-containing regions of the prostate gland showed increased blood flow, blood volume, and permeability 

 cautioned readers that blood volume and permeability could be measured precisely only in regions of increased tumor blood flow. Harvey et al observed that acute hyperemic response following radiotherapy resulted in an increased global prostatic perfusion at one to two weeks that was maintained in six to 12-week scans.

 Tumor oxygenation is an important prognostic factor for cervical carcinoma being treated with radiotherapy. Well-oxygenated tumor is radiosensitive, whereas hypoxia results in tumor radioresistance. Perfusion parameters were found to correlate with tumor oxygenation.

 Haider et al reported a moderate positive correlation of blood flow with tumor oxygenation in 32 patients with cervical cancer.

 A relationship between CT perfusion parameters and tumor and nodal staging was also observed. These findings indicate that perfusion CT has a potential role in risk stratification for cervical cancer.

 CT plays a key role in diagnosis and treatment monitoring of lymphoma. Morphologic CT criteria such as nodal size and appearance, however, show poor correlation with lymphoma grade and activity.

 As angiogenesis is not a predominant feature of lymphomas, perfusion imaging has not drawn the attention of researchers for tumor angiogenesis assessment. Dugdale et al evaluated 39 patients with lymphoma and found that median perfusion values of lymph nodes were higher in active disease and intermediate/high-grade lymphoma when compared with inactive disease and low-grade lymphoma, respectively.

 Blood flow substantially decreased when nodal disease became inactive, but there was no correlation of permeability with disease activity or grades of lymphoma.

It is important to emphasize that none of the perfusion CT acquisition or analytical techniques are standardized for implementation in routine clinical practice for oncologic body imaging, and all of the perfusion parameters calculated by commercial software are only estimates and not absolute values. One of the principal limitations of CT perfusion is the limited scanning volume. Current four- to 16-slice CT scanners allow 2-cm tissue coverage for dynamic CT acquisition. Even with a 64-slice scanner, the maximum coverage is only 4 cm. Hence, only a portion of the tumor or organ can be sampled for perfusion measurements. Scan volume selection is thus a crucial aspect of scanning technique.

Another issue is patient motion, which may be corrected by registration within the image plane. Patient motion out of image plane, however, causes data loss and errors in perfusion values. Respiratory gating may improve motion problems at the expense of temporal resolution.

 Adequate patient instruction for breath-hold, with immobilization straps over the abdomen, may help.

Beam-hardening artifacts from metallic stents, prostheses, and surgical implants can also result in variation in the perfusion values, and careful selection of region of interest for scanning is important here as well. Likewise, adequate distention of hollow viscus such as colorectum or stomach with saline or any neutral contrast is crucial for optimum perfusion measurements. In our institution, we use 250 to 300 mL of saline to distend the rectum before conducting perfusion CT studies.

Radiation dose is an important concern in implementing perfusion CT in routine clinical practice. Currently, the radiation dose is two to four times greater for perfusion CT than routine contrast-enhanced CT of the same region using the same scanning parameters, depending on the scanner.

Efforts to improvise further perfusion CT applications are ongoing. Respiratory gating, for example, would reduce misregistration breathing artifacts. Measurement of peak tissue enhancement can provide more physiological data by normalizing them for the patient's body weight and dose of contrast medium administered. Such an approach could determine the standardized perfusion value (SPV), defined as the ratio of tumor perfusion to mean whole-body perfusion.

 Mean whole-body perfusion is cardiac output divided by patient weight.

Miles et al reported that SPV measurements correlated with standardized uptake value measured on FDG-PET for lung nodules. The threshold of SPV to distinguish between benign and malignant lesions would be 1.5.

 The increasing availability of PET/CT raises the possibility of combining CT perfusion imaging with PET as well. This combined PET/CT/perfusion "triple" imaging could obtain anatomic, metabolic, and angiogenic information about tumor with a single scan.

Some vendors have recently introduced a table toggle technique to increase scan coverage and address current limitations in scanning volume in perfusion CT. In this technique, the scanner couch toggles between two locations for dynamic acquisition. This application is only possible, however, at the expense of temporal resolution, which would be five seconds instead of one second in static table technique.

Currently available iodinated intravenous contrast materials are limited by their short intravascular life. Further research should develop new agents with a longer intravascular life that could overcome these difficulties. Using automatic exposure control and low tube potential could optimize radiation dose during serial dynamic scanning.

Perfusion CT offers new opportunities for detection, risk stratification, and therapeutic monitoring of tumors. Although extensively researched in current literature, the clinical role of perfusion CT in the body is still evolving. Further research and improvements in acquisition and analytic techniques would open doors for perfusion CT to become a routine protocol for oncologic imaging and expand its clinical applications.

Dr. Namasivayam is a research fellow, and Dr. Sahani is director of CT, both at Massachusetts General Hospital in Boston. Dr. Sahani has received grants and research support from GE Healthcare.

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The most crucial process to assess in a tumor for efficient management is neo-angiogenesis. Currently, no reliable biomarker is available to measure angiogenesis, and direct microvessel density assay requires frequent biopsies for serial tumor monitoring. Technological innovations in imaging, however, have enabled the capture of physiological information in addition to anatomic details. One promising imaging technique for acquiring physiological information about angiogenesis is perfusion CT, which provides anatomic information as well.

Routine body perfusion CT approaches clinical reality

The prevalence of end-stage renal disease (ESRD) has increased over the last decade. Combined with a limited supply of cadaver kidneys, the increase has almost quadrupled the waiting list for kidney transplantation.

 This mismatch between the demand and supply of kidney grafts has led to a tremendous rise in the number of renal transplant surgeries with kidneys procured from living donors.

Use of the laparoscopic approach to harvest kidneys from living donors is also growing, as this approach lowers morbidity and shortens hospital stays compared with open nephrectomy, leading to a high degree of patient satisfaction. Laparoscopic surgery is technically challenging, however, due to limited surgical exposure and poor operative visibility.

 Detailed evaluation of the kidney donor with imaging is essential prior to such surgery to decrease intraoperative morbidity and complications.

Since the introduction of the spiral scanner, CT angiography has been extensively used in the body as a noninvasive alternative to catheter angiography. Results from single-slice spiral CT scanners for the assessment of living renal donors have been encouraging.

 Multislice CT offers several advantages over single-slice scanners and is therefore well suited for this indication.

Depending on the number of channels (four, eight, 16, and so on), MSCT scanners are four to 25 times faster than conventional single-slice spiral scanners. The shortened scan duration effectively reduces motion artifacts and allows scanning in the optimal arterial and venous phases separately. In addition, MSCT allows longer z-axis coverage, as tube heating is no longer an issue. Finally, MSCT offers better z-axis resolution, producing near-isotropic imaging data, which helps to create excellent-quality multiplanar reformations and 3D rendering.

MSCT evaluation of kidney donors is technically challenging. Optimal timing of image acquisition after contrast administration is crucial because the scan times are very short (@7 seconds on 16-slice CT scanners). It is highly advisable to individualize the scan delay for arterial phase instead of using an empiric scan delay. This can be performed by injecting a test bolus to estimate the time of arrival of contrast agent in the arteries. Most current MSCT scanners offer automated bolus-tracking capability that enables initiation of scanning automatically in the artery of interest at the desired threshold of arterial enhancement. 

At our institution, we use an automated triggering technique, placing the region of interest in the aorta and defining the threshold of aortic enhancement at 125 to 150 HU. We routinely use about 120 to 130 cc of iodinated nonionic contrast medium (300 mgI/mL), injected at a rate of 4 mL/sec through a peripherally placed 18 to 20-gauge IV cannula (see table).

Some investigators have proposed that optimal angiographic enhancement can also be achieved with the use of a low volume (80 mL) of high iodine concentration contrast media (400 mgI/mL). A saline flush after the administration of contrast medium has also been found useful. The field of scanning should include the origin of the celiac artery and the bifurcation of the iliac arteries, as the accessory renal arteries can originate from celiac, superior mesenteric, and common iliac arteries.

Each data set (arterial phase, venous phase, and excretory phase) is retroreconstructed with a 50% overlap for image postprocessing. A dedicated 3D workstation and trained technologists are imperative to achieve proper image reconstruction in various orthogonal planes and to create multiplanar reformations and 3D rendering.

 We routinely create coronal reformations, multiplanar subvolume maximum intensity projections (MIPs), and volume-rendered images. 

Axial source images are the basis for making diagnosis, and these are reviewed on a workstation. As the number of images can be 300 to 600 per patient, viewing films is impractical. A dedicated workstation allows rapid scrolling through the images and provides tools for generating various measurements of donor anatomy. Noncontrast images are useful for detecting renal or urinary tract stones and for characterizing incidentally detected renal or adrenal tumors. Arterial phase images are evaluated for presence, location, and number of renal arteries and their branching pattern.

Multiple renal arteries may be seen in 25% to 30% of donors (Figure 1). These accessory arteries may arise from the aorta or common iliac arteries and, rarely, from celiac and superior mesenteric arteries. They are smaller than the main artery. An early branching (within 1.5 cm from the origin) of the main renal artery is important to recognize,7 because most surgeons require a length of renal artery before branching of at least 1.5 cm for proper anastomosis in the recipient. Bilateral occurrence of multiple renal arteries has been reported in up to 15% of individuals.

Identification of renal artery stenosis or fibromuscular dysplasia in the donor renal arteries is equally important. And the presence of a renal tumor or a congenital anomaly such as crossed fused ectopia, horseshoe kidney, or medullary sponge kidney is an absolute contraindication for donor nephrectomy.8-11 The presence of simple cysts, however, is not considered a contraindication. The presence of kidney stone(s) is a relative contraindication, and guidelines may vary among institutions and transplant surgeons.

Knowledge of donor venous anatomy is crucial prior to undertaking a laparoscopic surgery to avoid inadvertent injury to the accessory renal veins and their tributaries. The left kidney is usually preferred for the open approach as well as for the laparoscopic approach because of the longer left renal vein. Multiple renal veins are encountered less frequently than multiple arteries. When present, accessory renal veins usually occur on the right side (Figure 2).

Other critical donor venous anomalies that may preclude donor nephrectomy on that side include a retro-aortic left renal vein, circumaortic left renal vein, and renal arteriovenous malformation/fistulae.

 Identification of anomalous venous drainage of adrenal, lumbar, and gonadal veins is important to minimize vascular injury during laparoscopic surgery.

The excretory phase images are assessed for the presence of urinary tract duplication, hydronephrosis, or any ureteric pathology. Though simple duplication of a ureter (Figure 3) is not an absolute contraindication for donor nephrectomy, it affects the surgical technique for ureteric implantation.

In addition to the evaluation of the kidney and urinary tract, the other abdominal organs should be evaluated for the presence of incidental pathologies that may preclude a donor nephrectomy or may affect selection of surgical procedure.

ACCURACY OF MSCT IN RENAL ARTERY EVALUATION

The accuracy of MSCT for the detection of accessory renal arteries has been shown to range from 89% to 97%.

 A small accessory artery missed on MSCT is usually due to interpretation errors and rarely from nonvisualization of the artery. Careful review of axial and reformatted images can minimize these errors. 

Three-D imaging with MSCT has come to the forefront in the evaluation and selection of open versus laparoscopic approaches to harvesting the donor kidney. The exquisite detail provided by MSCT prepares a transplant surgeon for what to expect during the surgery and thereby minimizes unexpected intraoperative complications that may jeopardize the outcome of a renal graft. Although it is critical that the radiologist be familiar with the transplant surgeon's requirements, the resolution provided by thin-section MSCT and the quality of 3D images have partly eliminated the need for a reader with extensive experience to interpret these dedicated exams.

MSCT is highly accurate in evaluating renal donors. A proper CT technique and careful review of CT data are important to provide the highest levels of accuracy in evaluating renal donors by MSCT.

DR. SAHANI is a clinical instructor of radiology at Harvard Medical School and an assistant in radiology at Massachusetts General Hospital in Boston. DR. KALVA is a clinical fellow in the department of vascular imaging and intervention at MGH.

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The prevalence of end-stage renal disease (ESRD) has increased over the last decade. Combined with a limited supply of cadaver kidneys, the increase has almost quadrupled the waiting list for kidney transplantation. This mismatch between the demand and supply of kidney grafts has led to a tremendous rise in the number of renal transplant surgeries with kidneys procured from living donors.

Dushyant Sahani, MD

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