Leonard M. Freeman

Don't Bury the V/Q Scan: It's as Good as Multidetector CT Angiograms with a Lot Less Radiation Exposure

Within the past decade, advances in CT technology have allowed superb angiographic studies for pulmonary embolism (PE) diagnosis. This has been embraced by clinicians and most diagnostic radiologists who are more comfortable with an anatomic demonstration of whether a clot is present or not as compared with looking at ventilation–perfusion (V/Q) mismatches on a lung scan (1). As a result, the number of multidetector CT angiograms (MDCTA or CTA) performed in the past few years has greatly increased with little regard for radiation dose, particularly to the breasts of young women. In addition, sufficient evidence has now been accumulated to confirm that V/Q scintigraphy is as accurate as CTA, making it a viable alternative when studying patients for PE. In a recent Invited Perspective in The Journal of Nuclear Medicine, Dr. Arnold Strashun comments on the MDCTA versus V/Q issue and concludes that ‘‘were it not for definite allergic and nephrotoxic risks of contrast media and the added radiation burden of MDCTA, the ventilation/perfusion scan would virtually disappear from the diagnostic algorithm for pulmonary embolism’’ (2). The CTA problems mentioned by Strashun are innate to the modality and cannot be mitigated. Therefore, it appears quite clear that V/Q imaging will continue to be a very important part of the PE diagnostic algorithm for the foreseeable future. In addition to substantial data confirming that V/Q sensitivity is comparable to that of MDCTA, the radiation risk aspect (particularly to the female breast) requires much stronger emphasis. Young women represent a very significant segment of the population being studied. As pointed out in the recent American College of Radiology (ACR) white paper (3), dose calculation is very complex because absorption in each organ is very variable from patient to patient. Breast radiation estimates made using 4-slice CT vary from 20 to 60 mSv (4–6), whereas that from V/Q is approximately 0.28–0.9 mSv (7). A recent report by Einstein et al. (8) estimated that 64-slice chest CTA delivers a dose of 50–80 mSv to the breast. These reports indicate an enormous 65to 250-fold difference between the 2 procedures. Average estimates generally quoted are a 70to 100-fold difference. Comparatively, a 2-view mammogram is associated with 3 mSv (4), which makes the CTA radiation dose approximately 10 to 20 times greater. In addition, the estimated radiation exposure from CTA suggests that a nonnegligible increase in lifetime attributable risk of cancer exists, particularly to the breasts of young women (1 in 143 for a 20-y-old woman and 1 in 284 in a 40y-old woman) (8). The International Commission on Radiation Protection (ICRP) has reported that CT doses can exceed limits shown to result in an increase in cancer risk (9). These are facts about which we all must be very concerned and be professionally obligated to deal with them. To address this great concern, the ACR white paper (3) strongly emphasizes that it is the responsibility of the imaging physician to be fully educated concerning the radiation risks associated with each procedure and, in turn, educate the clinician requesting the procedure. Presenting them with diagnostically equivalent options also is part of this educational process. This is certainly of paramount importance and pertinence in the MDCTAvs. V/Q issue. One advantage of CTA over V/Q mentioned by Strashun is that a significant number of patients have alternate anatomic diagnoses made that may be the cause of the patient’s symptomatology (2). Most of these serendipitous findings, such as small lung nodules, generally are not related to the patient’s acute problem. However, occasional significant findings, such as dissecting aneurysms or pneumothorax, may be detected (10). In addition to the most important radiation issue, there are several factors that should be considered when deciding between a CTA and V/Q study. These are (a) Is one of the studies clearly superior to the other in the overall context of diagnosing PE as well as in combination with objective clinical assessment (pretest probability)? (b) Are both studies equally available on a 24-h/7-d schedule? (c) Do the physicians interpreting the study possess the appropriate expertise?

Preoperative parathyroid localization with sestamibi

PURPOSE Results of noninvasive preoperative parathyroid localization with technetium99m-labeled sestamibi are reported in a series of 51 patients. PATIENTS AND METHODS Forty-four patients had hyperparathyroidism surgically treated for the first time and seven patients underwent reexploration for recurrent or persistent hyperparathyroidism. Preoperative scintigraphy with sestamibi was performed in all patients before surgical exploration. Results of the radionuclide studies were compared with surgical and pathologic findings. RESULTS Twenty-six patients had solitary adenomas. All 26 were localized preoperatively by the scans. Among 18 patients with multiglandular pathology, 69 pathological glands were found at surgery. Thirty-six of these glands, in 15 patients, were localized by the scans. Among the 7 patients evaluated after failed exploration or recurrent hyperparathyroidism, 7 pathologic glands were found, of which 6 were correctly localized by the scan. Ectopic lesions in 2 patients were correctly localized by the scan. CONCLUSIONS The authors conclude that sestamibi parathyroid localization is an effective method for preoperative parathyroid localization, with accuracy exceeding that of other noninvasive studies.

Cholescintigraphy, ultrasonography and computerized tomography in the evaluation of biliary tract disorders

Newer modalities available for the evaluation of hepatobiliary disease include cholescintigraphy, ultrasonography, and computerized tomography. We have examined the relative strengths and weaknesses of each of these noninvasive techniques and developed a rational diagnostic approach for the evaluation of acute cholecystitis, chronic cholecystitis, and cholestasis. The procedure of choice for suspected acute cholecystitis is 99m Tc-HIDA cholescintigraphy because it is a highly accurate method for obtaining functional information with regard to cystic duct patency. In suspected chronic cholecystitis, the oral cholecystogram is the best screening procedure, followed by ultrasound for confirmation of gallbladder disease as the cause of nonvisualization. The role of 99m Tc-HIDA cholescintigraphy in suspected chronic cholecystitis is limited to those cases where the oral cholecystogram and sonogram yield disparate results, or where a patient is known to have chronic gallbladder disease and superimposed acute exacerbation is suspected. Ultrasonography is recommended as the initial procedure for evaluation of the patient with cholestasis. It is highly accurate in distinguishing hepatocellular disease from obstructive jaundice, and when dilated biliary radicles are visualized, ultrasonography is frequently capable of identifying the cause of obstruction. If the patients body habitus or gaseous distention makes ultrasonographic evaluation difficult, then computerized tomography is recommended, followed by endoscopic retrograde cholangiopancreatography or transhepatic cholangiography, when needed.

The current and continuing important role of ventilation-perfusion scintigraphy in evaluating patients with suspected pulmonary embolism.

After the publication of the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study in 1990, there was considerable controversy concerning the ventilation-perfusion (V/Q) study in regard to its low sensitivity and high number of nondiagnostic examinations when used in patients with suspected pulmonary embolism (PE). Many lessons have been learned from the PIOPED database that have greatly improved our interpretive skills in the 2 decades since the study was performed. One of the key problems negatively impacting interpretation was the predominantly inpatient population that was studied. Inpatients generally are sicker patients with abnormal chest x-rays. This factor significantly degrades V/Q interpretation. A normal chest x-ray greatly facilitates accurate interpretation of the lung scan. The emergence of computed tomography angiography (CTA) in the early to mid-1990s provided a superb new means of imaging patients with suspected PE. As this technology became more sophisticated with multidetector units, it became the procedure of choice in the great majority of medical centers. CT scanners located in or proximal to many emergency departments as well as its 24/7 availability supported this preference. Within the past 2 to 3 years, the publication of the PIOPED II study as well as some other prospective and retrospective studies have confirmed similar diagnostic accuracy for CTA and V/Q studies. Additionally, there have been several recent publications cautioning physicians about the large radiation dose associated with CTA, particularly to the female breast. Considering the great benefits of both techniques as well as their limitations, it is prudent for both clinicians and imaging physicians to develop an appropriate approach to studying patients with suspected PE. Considerations such as objective clinical assessment, D-dimer assay and the chest x-ray appearance all play significant roles in this decision-making process.

Radiopharmaceuticals for renal studies

A review of the wide spectrum of radiopharmaceuticals developed over the last 15 yr for application in the evaluation of diseases of the kidney and urinary tract is presented below. The radiolabeled contrast agents including Hippuran have been extensively applied for renal function assessment. The kinetics of clearance of many of these agents are not yet satisfactorily established, and there is no ideal agent for routine clinical application, particularly by external counting methods. Many agents for intrarenal blood flow measurement have been reported that have yet to be adapted for obtaining clinical information. Renal morphology has been studied using several radiopharmaceuticals that are either fixed in the renal tubules for a sufficiently long time (chlormerodrin, 99m Tc-Fe ascorbic acid) or cleared from the kidneys fairly rapidly (Hippuran). These agents suffer from several disadvantages because of suboptimum energy of the radiolabel for use with imaging equipment, or relatively high radiation dose, or slow rate of excretion with inadequate organ specificity. It is hoped that the development of new radiopharmaceuticals tagged with radionuclides that have ideal imaging characteristics ( 99m Tc or 111 In) and satisfactory renal clearances will provide a major breakthrough in the continuing search for satisfactory renal agents for imaging and function tests in the near future.

The Role of Radionuclide Imaging in Spleen Trauma

Sixteen patients with abdominal trauma and possible spleen injury were screened by radionuclide imaging. A multiple-view negative examination appears to weigh heavily against any significant splenic injury requiring surgical intervention. Angiography probably is not needed if the spleen scintigram is negative. False positive examinations may be minimized by performing oblique and angulated views, bearing in mind the potential existence of normal grooves and clefts such as rib imprints. Unexplained bands and areas of diminished activity may be encountered. Careful correlation with the patients history, physical findings, and clinical course should clarify such problems. Angiography may be necessary in many of these cases.

Evaluation of the postoperative patient with 99mTc-IDA cholescintigraphy.

In order to assess the role of 99mTc-iminodiacetic acid (IDA) cholescintigraphy in evaluating postoperative patients, a total of 213 studies were performed in 189 patients over a 3-year time period. Of these, 130 studies were obtained in 125 cases with signs and/or symptoms suggesting postcholecystectomy syndrome. A normal sized duct that emptied within an hour ruled out significant pathology with a high degree of accuracy (97%). A less reliable finding of normalcy was the combination of ductal dilatation with functional patency in that three of 20 patients (15%) who exhibited this pattern were proven to have nonobstructing calculi in their common bile duct. AZ spectrum of abnormal findings was encountered. Ductal dilatation was a most significant indicator of partial or intermittent ductal obstruction when it was associated with altered time-activity dynamics in the ducts and secondarily, delayed biliary-to-bowel transit time of the radiotracer. Patterns indicating complete common duct obstruction, cystic duct remnants, and bile leaks also proved to be very sensitive. Seventy-three studies in 56 patients very accurately evaluated the integrity of biliary-enteric bypass anastomosis. Complete and partial obstructive patterns were similar in appearance to those encountered in postcholecystectomy syndrome. Several leaks were also detected in this patient population. Ten studies were performed in eight patients who underwent Billroth II gastroenterostomies primarily to see if afferent loop obstruction was present. Three of these patients did demonstrate dilated A-loops with stasis, thereby making a positive diagnosis possible.

False positive liver scans caused by disease processes in adjacent organs and structures

Abstract As a result of its great pliability, the liver can change its shape and form when impinged upon by disease processes in neighbouring organs and structures. The resultant hepatic scan is quite confusing, since the picture portrayed is frequently indistinguishable from that of intrinsic space-occupying disease of the liver. Enlarged gall-bladders, kidney cysts and tumours, pancreatic lesions, dilated bile ducts and subdiaphragmatic fluid are examples of extrahepatic disease processes that can simulate intrahepatic defects. It is quite essential that the physician interpreting hepatic scans has an appreciation that such a situation can exist. In this manner, one may hopefully reduce the incidence of his “false positive” interpretations. When doubt exists on the hepatic scan, a careful consideration of the clinical and laboratory findings, as well as the employment of complementary radiographic techniques such as selective angiography, can help elucidate the problem.

Nonosseous, Nonurologic Uptake on Bone Scintigraphy: Atlas and Analysis

Uptake in nonosseous, nonurologic tissues is occasionally found in the performance of bone scintigraphy. Proper interpretation of these cases depends on identifying the involved organs and appreciating the significance of the uptake. Because of the rarity of these findings and a relative de-emphasis of planar imaging in radiologic imaging, current era trainees may exhibit difficulty in identifying organs on planar projections. The first section of this work consists of an image atlas depicting uptake by various nonosseous, nonurologic organs on planar scintigraphy. In the second section, we discuss the etiologies of soft-tissue uptake, organized according to mechanisms of accretion: (1) metastatic calcification, (2) dystrophic calcification, (3) metabolic uptake, and (4) compartmental sequestration. Spurious or artifactual uptake represents a fifth category of apparent soft-tissue uptake which will also be reviewed. Causes of organ uptake span the gamut of trivial and artifactual to those reflecting serious malignant and systemic disorders. Clues as to etiology may be gleaned from grouping of abnormalities.

Radionuclide venography: Its place as a modality for the investigation of thromboembolic phenomena

The radionuclide venogram graphically depicts thrombotic disease and appears particularly helpful in the visualization of the external pelvic venous system, inferior vena cava, and veins of the thigh. Since the pulmonary scintigram also may be performed in conjunction with the radionuclide venogram, the patient also is screened for possible pulmonary emboli. The extent of thrombotic disease in the area imaged by radionuclide venography appears to be related to the incidence of pulmonary emboli. It is likely to image nonocclusive thrombi, those which are more likely to embolize. It provides information regarding the status of venus channels, varicose veins and venous insufficiency, in addition to the presence of thrombi. Several authors have shown a better than 90% correlation between the findings on radionuclide venography and those of the most widely accepted means of assessing thrombosis in leg veins, radiographic phlebography. The technique and clinical results of radionuclide venography as well as its advantages and disadvantages in comparison to radiographic phlebography are discussed here. A simplified, standardized method of performing radionuclide venography with a moving-table accessory is described and illustrated.