Diagnostic nuclear medicine in the ED

American Journal of Emergency Medicine (2011) 29, 91–101

www.elsevier.com/locate/ajem

Review

Diagnostic nuclear medicine in the ED Behrang Amini MD, PhD a,⁎, Chirag B. Patel MSE b , Matthew R. Lewin MD, PhDc,⁎, Taegyeong Kim BSd , Ronald E. Fisher MD, PhD e a

Department of Radiology, Tufts Medical Center, Boston, MA, USA Department of Diagnostic and Interventional Imaging, University of Texas Health Science Center at Houston School of Medicine, Houston, TX, USA c Department of Emergency Medicine, University of California, San Francisco, San Francisco, CA, USA d University of Houston College of Pharmacy, Houston, TX, USA e Department of Radiology, The Methodist Hospital and Baylor College of Medicine, Houston, TX, USA b

Received 17 December 2008; revised 4 March 2009; accepted 5 March 2009

Abstract Although the decision to use nuclear medicine (NM) modalities in the acute care setting is limited by several factors, there are instances in which the use of NM techniques can provide elegant and efficient solutions to otherwise expensive and resource consuming situations. Herein, we describe the indications and NM techniques used for the evaluation of low-risk patients with chest pain, suspected pulmonary embolus, acute cholecystitis, gastrointestinal bleeding, acute scrotum, and the radiographically occult fracture. © 2011 Elsevier Inc. All rights reserved.

1. Introduction Nuclear medicine (NM) is a specialized branch of medical imaging practiced by residency trained NM specialists and, in limited circumstances, radiologists. Nuclear medicine techniques use unsealed radioactive substances in diagnosis and therapy. In diagnostic NM, radioactive substances are administered, usually intravenously, to patients, and the radiation emitted is measured. Nuclear medicine imaging primarily shows the physiological function of the system being investigated while also providing low-resolution anatomical data. There is often confusion about the diagnostic use of NM in the emergency department (ED). The use of NM in ⁎ Corresponding authors. Tufts Medical Center, Department of Radiology, Box# 299, Boston, MA 02111, USA. E-mail addresses: [email protected] (B. Amini), [email protected] (M.R. Lewin). 0735-6757/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.ajem.2009.03.008

the ED is often limited by time and staffing constraints, prompting the use of other radiographic techniques. The NM study with the most utility and most often used in the ED is myocardial perfusion imaging (MPI), which has been shown to reduce costs, decrease the length of hospital stays, and lead to more rapid diagnosis [1]. Nevertheless, MPI remains underused in the ED. Ventilation/Perfusion (V/Q) scanning, although largely replaced by high-resolution computed tomography (CT) for the diagnosis of pulmonary embolism (PE), has a role in patients who cannot receive intravenous (IV) contrast, in young women where there is concern about possible carcinogenic effects of radiation on the breasts, and particularly in pregnant patients with the concern of fetal radiation dose. The use of NM in the emergent diagnosis of cholecystitis, gastrointestinal (GI) hemorrhage, acute scrotum, and occult fractures is also reviewed. We review the use of NM in the ED, highlight cases where NM remains useful, and describe its limitations.

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2. Discussion 2.1. Acute coronary syndrome Approximately 50% of the 5 million patients who present to EDs in the United States with acute coronary syndromes (ACSs) end up with costly hospital admissions [2,3]. Only 10% to 15% of patients admitted have documented myocardial damage or unstable angina [4,5], whereas up to 10% of discharged patients have an unrecognized myocardial infarction (MI) [6-11]. Patients discharged with an unrecognized MI have a mortality rate almost twice that of those initially admitted [6,7]. This high-cost, low-efficiency clinical practice, together with the high legal risk it carries, has driven liberal admission policies that cost $10 to $13 billion per year to rule out MI in low-risk patients [3,12]. These factors stress the need for rapid risk stratification of patients with ACS to guide the decision to admit the patient. The negative predictive value of resting single photon emission CT (SPECT) sestamibi imaging for ruling out MI is up to 99% [13-16], suggesting that MPI is valuable in deciding to admit or discharge a patient from the ED. In addition, MPI can lead to quicker decisions regarding the presence of acute MI. Initial cardiac troponin I values drawn at the same time as MPI have been shown to have a sensitivity of only 39% compared to 92% for MPI [17]. It is only after 24 hours that the sensitivity of cardiac enzymes approaches that of acute rest SPECT sestamibi imaging obtained when the first cardiac markers were drawn [17]. Thus, MPI can identify patients with ACS much earlier than enzyme markers. The specificity of MPI, however, was lower than serial cardiac troponin I levels (63%-71%, compared to 94%98% when a cutoff of 2.0 ng/mL was used). In addition, acute resting MPI cannot differentiate between acute infarction, acute ischemia, or old infarction (see below), whereas serial cardiac troponin I levels can identify patients with myocardial necrosis. Therefore, serial cardiac troponin I levels and MPI should be thought of as complementary examinations [17]. Depending on the institution, cardiac nuclear studies in an acute setting can be ordered by the emergency physician. The nuclear study often can be performed and the patient discharged without consultation from a cardiologist. Myocardial perfusion imaging, as performed in the ED, involves IV injection of a radiotracer, generally 99mtechnetium (Tc)-sestamibi or tetrofosmin, followed about 20 to 60 minutes later by SPECT imaging of the heart. Delays are introduced related to camera availability, arrival of a nuclear technologist (for after-hours studies), and sometimes, delivery of the radiotracer to the hospital. Thus, a result will usually be available approximately 2 to 4 hours after the test is ordered, although the wait will vary widely among hospitals. The resting perfusion study can demonstrate infarcts and acute ischemia and can sometimes distinguish the 2 using gated SPECT [18]. It cannot, however, distinguish between acute and old infarcts. It is not a complete stress test but can be combined with an exercise or

B. Amini et al. pharmacological stress perfusion study the next day. Examples of a normal perfusion image, a resting image from a patient with acute chest pain, and a second-day stress image from the same patient are shown in Fig. 1A-C. When compared with strategies that do not use MPI, investigative strategies using MPI are cost-effective and have the same outcome [1,19,20]. The reduced expenses are due to decreased usage of angiography and shorter median hospital and intensive care stays. Because almost two thirds of the 4 million patients who visit the ED for chest pain each year present with nondiagnostic electrocardiogram (ECG) findings [8,10,21-23], triage decisions in the ED can be

Fig. 1 Cardiac imaging. A, Short-axis view from a healthy subject looks like a doughnut, with the anterior, lateral, septal, and inferior walls as labeled. Note that only the left ventricle is well seen on a myocardial perfusion study because it is much thicker than the right. B, Short-axis view from a patient with acute chest pain performed at rest shows a large, severe perfusion defect in the inferior and inferolateral walls, consistent with an infarction. C, Short-axis view form an adenosine stress perfusion study performed the next day shows that the inferior defect is even larger and thus represents an infarction with associated ischemia. In addition, stress images show involvement of the anteroseptal, anterior, and anterolateral areas (arrows) not seen on the rest images, indicating the presence of reversible ischemia in the LAD territory. D, A reconstructed image from CT angiography in a patient with acute chest pain shows a 90% stenosis of the right coronary artery (arrow) just proximal to the acute marginal branch. A calcified (hard) plaque is seen just distal to the stenosis. The CT slice oriented to optimize visualization of the right coronary artery. E, Three-dimensional rendering of the images shows a 90% stenosis of the right coronary artery (arrow).

Diagnostic nuclear medicine in the ED improved by including quality MPI for patients with negative or nondiagnostic ECGs and suspected ACS [24]. Patients benefiting the most from nuclear imaging are those without prior MI and with nondiagnostic or normal ECGs and symptoms suspicious for acute ischemia. Patients with previous MIs will often have a perfusion defect representative of the old infarction, making resting MPI less helpful for discrimination of a new MI [24]. Rapid CT scanning with multidetector CT (MDCT) is an exciting new technique for performing noninvasive coronary angiography [25-28]. It provides additional prognostic information by assessing coronary calcifications (coronary calcification score). Although results from large-scale trials have not yet been published, smaller studies indicate a very high sensitivity for diagnosing significant coronary artery disease in patients presenting with acute chest pain in the ED [29,30]. One direct comparison with sestamibi SPECT imaging showed comparable to slightly better accuracy in diagnosing ACS with MDCT compared with MPI [29]. The negative predictive value of both tests was extremely high in the acute chest pain patient (97% with MPI and 99% with MDCT). Multidetector CT angiography potentially has a significant advantage in the ED in that a diagnostic result will generally be available quite rapidly, particularly if such scanners are placed in the ED. Potential disadvantages of MDCT include a rather high radiation dose, particularly to the female breast, and the necessity of reasonably good renal function to allow the use of a large volume of IV contrast. An example of CT angiography is provided in Fig. 1D and E. This patient presented with angina, and the images clearly show a 90% stenosis of the right coronary artery. Very recently, hybrid SPECT-CT scanners with MDCT have become commercially available, and trials are underway to test the efficacy of an exciting new approach to patients with chest pain: combined MPI and MDCT scanning in a single scanning session. This would allow functional imaging of myocardial perfusion, delineation of the coronary anatomy, and a coronary calcification score, all to be obtained simultaneously. Although the potential power of this approach is obvious, it remains to be seen if the information from the 2 modalities is synergistic or redundant.

2.2. Pulmonary embolism Although clinical signs and symptoms allow the clinician to determine the pretest probability of a patient with PE, they are insufficient to diagnose or rule out the condition [31-34]. The diagnosis of PE can be aided by the D-dimer assay, pulmonary arteriography, CT pulmonary angiography (CTPA), and V/Q scanning. Despite recent advances, however, mortality from pulmonary embolic disease has remained unchanged during the past 25 years [35,36]. Examples of a normal V/Q scan and a high-probability V/Q scan are shown in Fig. 2. In V/Q scanning, ventilation images are usually acquired first with the patient breathing about 10 to 20 mCi (370-740 MBq) of Xenon-133 (133Xe)

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Fig. 2 The V/Q scans. A, Normal perfusion images in anterior (left) and posterior (right) projections. The corresponding normal ventilation images are not shown. B, A high-probability scan shows multiple, bilateral, moderate-to-large perfusion defects on the anterior (left) and posterior (right) views. C, Posterior projections of the ventilation scan accompanying the high-probability scan are normal. Thus, the perfusion defects in panel (B) are mismatched.

gas mixed with room air from a mask that is attached to a rebreathing machine. Dynamic imaging of the lungs is performed for 5 to 10 minutes while the xenon gas enters the lungs and then washes out of the lungs. Immediately after the ventilation study, the patient is injected intravenously with about 5 mCi (185 MBq) of 99mTc macroaggregated albumin (MAA) particles. The lungs are then imaged in 6 to 8 different projections. Anterior and posterior views are shown in Fig. 2. Normal perfusion images are shown in Fig. 2A, which excludes PE with greater than 98% confidence. Perfusion images in Fig. 2B demonstrate multiple, bilateral defects. When compared to the normal ventilation in Fig. 2C, this indicates a very high probability of PE. After formulating the pretest clinical probability of PE in a patient, CTPA is usually the next test performed. Although CTPA has essentially replaced V/Q scanning as the standard first-line test for PE [37,38], specific circumstances may allow alternatives to this approach. When deciding between V/Q scans and CTPA, a number of factors must be taken into

94 consideration. The availability of physical and human resources at the medical facility usually dictate the choice of diagnostic test, and it must be noted that patients with obstructive pulmonary disease and those with abnormal chest radiograph results are more likely to have nondiagnostic V/Q scans [39]. Because most patients with chest pain suggestive of PE end up with other or nonspecific diagnoses, CTPA is useful in that it allows for the determination of other pathologies. Indeed, in 1 study, CTPA found alternative diagnoses in more than 50% of patients evaluated by CTPA for PE [40]. Nevertheless, certain patient populations are of special concern when it comes to the use of ionizing radiation, most notably, those patients who are pregnant. In addition, iodinated contrast may be a potential concern in pregnant and breast-feeding patients, as discussed below, and is generally contraindicated in patients with poor renal function. In pregnant women, the decision of which test to use usually centers around the issue of fetal radiation exposure (Table 1). In pregnant women, a modified CTPA carries a very low radiation exposure to the fetus as long as the scans extend no further than 5-mm below the xiphoid process [41]. The worst estimated absorbed dose in a modified CTPA for the fetus is during the third trimester and is 0.13 mGy (0.013 rad), approximately 18 times less than the yearly natural background radiation [41,42]. This calculation is quoted in many articles and widely on the Internet and has formed the basis of many hospital decisions to use CTPA instead of V/Q scanning during pregnancy. Ventilation/Perfusion scanning also delivers a fairly low fetal exposure, although higher than CTPA (assuming CTPA does not extend significantly below the xiphoid process). Many institutions use a low tracer dose when performing V/Q scans on pregnant women, which further lowers fetal exposure. Images are then acquired for a longer time to ensure comparable counts in the images and thus comparable image quality. We have not been able to find survey data on the anatomic regions included in CTPA protocols across different institutions. It is critical to note, however, that the standard for CTPA is derived from the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) II study, where scans start 2 cm below the lowest diaphragm (well below the xiphoid) and proceed cranially to the top of the lung apex [43]. In the absence of survey data showing that institutions deviate from the (PIOPED) II protocols, we recommend that emergency physicians familiarize themselves with the scan protocol at their institution and have an active dialogue with radiologists when caring for pregnant patients suspected of having a PE. First-trimester estimates of fetal radiation dose with standard CTPA protocols can be as high as 0.32 mGy (0.032 rad) [44]. A second issue that is not as frequently considered is the radiation exposure to the maternal breasts. Current CTPA protocols deliver a fairly significant radiation burden to the breasts, even more so when the breasts are enlarged by pregnancy (Table 1). The tissue dose for the breasts of

B. Amini et al. Table 1 Comparison of breast and fetal dose for different imaging modalities for the diagnosis of PE Examination

Breast dose (mGy)

Fetal dose (mGy)

Chest radiograph CTPA

0.09 10-60 [43-46]

Modified CTPA a

10-60 [43-46]

Traditional V/Q scan b

0.90-1.39 [48]

Reduced-dose V/Q scan c

0.26-0.31 [48]

Reduced-dose perfusion-only scan d

0.22 [48]

b0.01 0.32, first trimester [43] 0.003-0.02, first trimester 0.008-0.08, second trimester 0.05-0.13, third trimester [41] 0.61-0.92, first trimester 0.75-1.14, second trimester 0.60-0.91, third trimester [48] 0.17-0.18, first trimester 0.21, second trimester 0.16-0.17, third trimester [48] 0.16, first trimester 0.20, second trimester 0.16, third trimester [48]

Dose for a chest radiograph is given by way of comparison. a In a modified CTPA, the most caudal section is 5 mm inferior to the xiphoid process. b Traditional V/Q scan is performed with 370 to 740 MBq of 133Xe, rebreathed for 5 minutes with a 5-L spirometer volume followed by 148 to 222 MBq of 99mTc MAA. c Reduced-dose lung scintigraphy is performed with 370 to 740 MBq of 133Xe, rebreathed for 5 minutes with a 5-L spirometer volume followed by 40 MBq 99mTc MAA. d Reduced-dose perfusion-only scan is performed with 40 MBq 99m Tc MAA.

nonpregnant patients is between 10 and 60 mGy (1-6 rad) [44-47]. The risk to proliferating, enlarged and radiosensitive breasts of pregnant patients has not been determined but is likely to be higher [48]. In contrast, breast dose during a reduced-dose V/Q scan (see below) is at most 0.31 mGy (0.031 rad) or almost 200 times less than a CTPA [49]. There is a dearth of data on the safety of fetal exposure to iodine and subsequent thyroid and renal dysfunction, as well as its effects on developing organs. The available data, obtained when arteriography and amniography were performed on pregnant women, have not demonstrated any adverse effects on the fetus [33,48,50]. In the face of scant data, risks that occur in the general adult population are extrapolated to the fetal population [50]. In addition, routine postnatal thyroid screening is thought to be adequate in assessing for possible thyroid injury [50].

Diagnostic nuclear medicine in the ED Some clinicians choose to eliminate fetal ionizing radiation and contrast exposure altogether through the use of lower extremity ultrasound as a first screening test for deep vein thrombosis. It must be noted, however, that the use of lower extremity venous ultrasound has not been systematically examined in pregnant patients [51] and that not all deep vein thrombosis originate in the lower extremity. A Ddimer test may also be considered. The negative predictive value for excluding PE of a negative D-dimer in pregnant women was shown to be 99.6% in a study of more than 1000 patients at Brigham and Women's hospital [52]. Although the elevation of D-dimer levels during pregnancy reduce the utility of D -dimer levels in this group of patients, approximately 40% of pregnant women before the 30th week of gestation have negative D-dimer levels, and 25% have negative D-dimer levels before the 42nd week of gestation [53,54]. These findings support the use of D-dimer tests in pregnant patients suspected of having a PE and can avoid radiological imaging and the risks from ionizing radiation to the fetus and maternal breast. Because there are currently no studies in humans that show how much free iodide crosses the placenta, how long it remains in the fetus, or what its effect on the fetal thyroid or renal function might be, one might consider the use of V/Q scans in pregnant patients. Radiation dose from V/Q scans in pregnant patients can be minimized by reducing the radionuclide dose by 50% to 80%. As seen in Table 1, a reduced-dose V/Q scan decreases fetal exposure from between 0.61 and 1.14 mGy (0.061-0.114 rad) to between 0.17 and 0.21 mGy (0.017-0.021 rad) [49]. The fetal dose from the ventilation portion of the study is minimal as long as it is performed with 133 Xe gas (Table 1). Some institutions perform the ventilation portion of the study with aerosolized 99mTc-DTPA, in which case the dose from the ventilation portion is not negligible. It should also be noted that 2 of the major conditions limiting the diagnostic accuracy of V/Q scans, COPD and pneumonia, are rarely present in pregnant women. In addition to the risk to pregnant patients, PE is the leading cause of maternal death after a live birth [55]. Women who undergo a cesarean delivery may have a 3- to 9-fold greater risk of PE compared with those who deliver vaginally [56], although not all authors agree with this assessment [57]. Clinical suspicion of PE in patients during the postpartum period is justifiably vigorously investigated. However, this raises concern for passage of radioactive or contrast material into breast milk. The Administration of Radioactive Substances Advisory Committee recommends a 12-hour interruption time for breastfeeding for mature milk after the administration of up to 80 MBq (2 mCi) of 99mTc MAA [58], which seems reasonable. The potential risks from iodinated contrast media to the infant are unknown. Numerous studies have shown low levels (eg, 1.7 mg I/kg with iohexol and 0.78 mg I/kg with metrizoate) of iodinated contrast media in milk after IV and intrathecal delivery of water-soluble

95 agents [50,59-64]. Thus, the likelihood of toxicity is extremely low. Based on these data, the Contrast Media Safety Committee of the European Society of Urogenital Radiology recommends no interruption in breast-feeding after the administration of iodinated contrast material [50]. As with other foodstuffs, the taste of milk may be altered if it contains contrast medium. It is generally accepted that iodinated dyes limit the use of high-resolution CT in patients with poor renal function and those with previous adverse contrast reactions. Finally, we note that suspicion of seafood “allergy” as a reason to avoid iodinated contrast is based more on medical myth than fact and is not a sufficient contraindication to CT angiography. A relationship between the iodine levels in seafood and seafood allergy is part of medical lore. Although iodine levels in seafood are higher than in nonseafood items, the consumption of the latter exceeds that of the former by far, and there is no evidence that the iodine content of seafood is related to reactions to seafood [65]. Available data do suggest that seafood allergy increases the risk of a contrast-mediated reaction to the same degree as that in patients with self-reported allergies to fruits or in those with asthma [66]. Eighty-five percent of patients reporting seafood allergies will not have an adverse reaction to iodinated contrast [65]. Finally, there is no evidence that adverse skin reactions to iodine-containing topical antiseptics (eg, Betadine, Povidine) are of any specific relevance to administration of IV contrast agents [65,67]. A number of clinical algorithms have been devised in the diagnosis of PE. Including them is beyond the scope of this review, but the interested reader is encouraged to review the following references: Gotway et al [39], Hogg et al [68], Musset et al [69], Roy et al [70], and Wells et al [71].

2.3. Acute cholecystitis Biliary scintigraphy, or hepatobiliary iminodiacetic acid (HIDA) scanning, involves the IV injection of a radioactive compound, usually 99mTc-mebrofenin, which is transported into the liver like bilirubin. The abdomen is imaged for 1 hour while the radiotracer is extracted from the blood by the liver and excreted into the bile. The radiotracer follows the flow of bile into the gallbladder and small bowel, producing a noninvasive evaluation of the liver, gallbladder, bile ducts, and duodenum with both functional and anatomic information. Because there is a high likelihood of acute cholecystitis in cystic duct obstruction, the HIDA scan can be used to diagnose acute cholecystitis, which appears as a nonvisualized gallbladder with prompt filling of the common bile duct and duodenum (Fig. 3). When compared to ultrasound for the diagnosis of acute cholecystitis, HIDA has better sensitivity (88% vs 50%), specificity (93% vs 88%), positive predictive value (85% vs 64%), negative predictive value (95% vs 80%), and accuracy (92% vs 77%) [72].

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B. Amini et al. parenteral nutrition, increases the chances of false positive HIDA results [78].

2.4. Acute GI hemorrhage

Fig. 3 The HIDA scans. A, Normal scan showing good uptake of radiotracer by the liver with normal hepatobiliary excretion that moves promptly into the gallbladder and small bowel. The gallbladder should appear by 60 minutes and is easily seen in this patient by 15 minutes (arrowhead). B, Acute cholecystitis. Radiotracer uptake by the liver, excretion into the biliary tree, and movement into the small bowel are normal. Radiotracer does not enter the gallbladder despite continued imaging after morphine injection. This is 98% predictive of acute cholecystitis.

The HIDA scan can also be used to diagnose common bile duct obstruction, which typically results in radiotracer uptake by the liver but no movement of activity into the gallbladder or small bowel [73]. Computed tomography is being used more frequently for the diagnosis of acute cholecystitis. Unfortunately, there are no adequate studies comparing the sensitivity and specificity of HIDA and US with CT in diagnosing acute cholecystitis. The only study in the literature has limitations that preclude definitive conclusions in this regard [74]. Based on these results, one might consider a HIDA scan as the first diagnostic modality in patients with suspected acute cholecystitis, reserving ultrasound for the confirmation of the presence of gallbladder stones. Unfortunately, practical concerns, such as NM technologist availability, may limit the use of HIDA scans on nights and weekends and will nearly always add a significant delay compared to obtaining an ultrasound. In addition, abdominal ultrasound can detect a number of incidental findings, such as pancreatic neoplasms, liver metastases, and renal cell carcinoma [75-77], whereas HIDA can only detect obstructions in the biliary system. In addition, gallbladder stasis, seen in critically ill patients and in patients receiving

Acute GI bleeding may require immediate surgical intervention. Endoscopy is useful when GI hemorrhage is present in the proximal or distal GI tract. When the bleeding site is located elsewhere, however, or when severe bleeding makes visualization of the bleeding site difficult, alternative diagnostic techniques should be considered. In these cases, angiography may prove useful, and nuclear scintigraphy has also been used with success for more subtle GI bleeding (eg, patients with multiple areteriovenous malformations). Angiography, like endoscopy, possesses an important advantage over nuclear scintigraphy by often enabling rapid intervention upon diagnosis. Nuclear scintigraphy can detect bleeds in some of these difficult patients but has the disadvantages of only approximate localization and lack of a therapeutic component. The most commonly used NM study for the detection of acute GI hemorrhage is 99mTc-labeling of autologous blood. The technique consists of removing about 4 mL of the patient's blood and labeling the red cells in vitro with 25 mCi (925 MBq) of 99m Tc-pertechnetate for about 20 minutes, then reinjecting the blood into the patient. The abdomen and pelvis are immediately scanned for 1 hour, during which time a GI bleed is detected as activity moving in a linear or curvilinear fashion (inside the bowel) and appearing outside the blood pool and urinary structures. Thorne et al [79], in an experimental canine model, showed that if at least 3 mL of blood pools at a single site, bleeding rates as low as 0.1 mL/min may be detected. These numbers are often contrasted to 0.5 mL/min needed for detection by angiography, as originally described by Nusbaum and Baum [80]. Although these numbers appear to give a rough estimate for the relative sensitivity of angiography compared to NM, it must be noted that the studies were conducted in experimental animals under highly controlled circumstances [79,80]. Therefore, the relevance of these numbers to human patients, especially those in the acute setting, is of dubious value. In addition, the experiments were conducted in different laboratories across many years, making statements regarding relative sensitivity of the 2 methods useless. More recently, helical CT has been used in a swine model to detect active colonic bleeding at rates of 0.5 mL/min or less [81], although this has yet to be introduced into the clinical arena. Although NM studies are considered to be more sensitive than angiography [79,82], in the emergency setting, they are often not helpful in localizing the source of hemorrhage. Endoscopy and angiography are the first-line modalities because they enable more precise localization of the site of hemorrhage and, more importantly, rapid intervention upon diagnosis. In short, if a patient is stable enough to receive a nuclear study for a GI bleed, he or she is most likely stable

Diagnostic nuclear medicine in the ED enough to be admitted for further investigation on the floor rather than in the ED.

2.5. Acute scrotum Testicular isotope scanning with 99mTc-pertechnetate has been reported to have 90% to 100% specificity and 89% to 98% sensitivity for testicular torsion [83,84]. The study can be rapidly performed and exposes patients to low ionizing radiation levels. The technique involves injection of the isotope intravenously, followed by blood flow images of the scrotum, immediately followed by blood pool images. Testicular isotope scans can differentiate between epididymitis, which results in “hot spots” due to increased perfusion near the affected testicle, and testicular torsion, which results in “cold spots” due to decreased blood flow to the affected testicle. For patients presenting with acute scrotal pain, color Doppler sonography has quickly become the first-line test for differentiating among acute epididymitis, epididymoorchitis, and torsion of the testicular appendage [85,86]. Although acute epididymitis and epididymoorchitis are amenable to conservative management, testicular torsion requires immediate surgical intervention [87]. Easy availability, rapid diagnosis, absence of ionizing radiation, and identification of incidental findings make ultrasound the first choice for evaluating the acute scrotum in the ED, and this modality has largely replaced nuclear imaging techniques [88]. Nevertheless, most NM departments still offer testicular isotope scanning, which may be used in cases of equivocal ultrasound studies instead of exploratory surgery, if appropriate procedures exist in the hospital for efficient performance of this study.

97 scaphoid fractures and allowing for reduced immobilization time of the wrist [93,95]. Thus, for the patient who delays seeking medical care for several days and presents to the ED 3 or more days after initial trauma, bone scans may allow the physician to rule out a scaphoid fracture. In addition, bone scanning in these patients frequently detects other fractures of the wrist incidentally [93,95]. Bone scanning is more sensitive than radiography for detection of a variety of other fractures, including hip fractures, pelvic insufficiency fractures, and stress fractures. Ninety-five percent of these fractures become apparent on bone scan within 24 hours after injury in patients younger than 65 years, and 95% become apparent within 72 hours

2.6. Occult fractures Without radiographic evidence of fracture, patients with signs and symptoms consistent with an acute fracture present a diagnostic challenge. In some cases, nuclear bone scanning can be helpful. There is wide variation in the way wrist injuries are evaluated [89], and one example is the detection of scaphoid fractures. Acute scaphoid fractures are notoriously difficult to detect radiographically. Immediately after injury, up to 65% of scaphoid fractures are radiographically occult [90]. Occult fractures are often treated conservatively with immobilization and followed by serial radiographs, resulting in unnecessary cost, radiation exposure, inconvenience, and loss of productivity to the patient. Bone scans of the wrist performed less than 48 hours after injury may be falsely positive, possibly due to traumatic synovitis [91,92]. When carried out between 3 and 7 days after injury, bone scintigraphy had 92% sensitivity and 87% specificity [93]. Although the American College of Radiology guidelines do not recommend using bone scans for diagnosis in acutely injured patients [94], day 3 or later bone scans are useful for ruling out

Fig. 4 Sacral insufficiency fracture. Anterior and posterior images of a whole-body bone scan with 99mTc-MDP in a 75-yearold woman with acute low back and sacral pain after minor trauma. Radiographs were negative. Marked radiotracer uptake is present in the lower sacroiliac joints bilaterally with a linear band of radiotracer extending across the sacrum (arrow), diagnostic of an insufficiency fracture. The appearance is sometimes called a “Honda sign” because of the resemblance to the automobile's emblem.

98 in patients older than 65 years [96]. Occasionally, though, in elderly patients, it can take up to a week for the scan to be positive. Thus, a negative bone scan 2 days after a fall in an elderly patient does not exclude a fracture; the scan would need to be repeated at 7 days posttrauma. An example of a pelvic insufficiency fracture in a 75 year-old woman with pelvic pain and negative radiographs 3 days after minor trauma is shown in Fig. 4. The bone scan unequivocally demonstrates an insufficiency fracture that involves both sacroiliac joints and the intervening sacrum, a typical finding in such fractures. The use of bone scanning is increasing for the purpose of estimating the age of vertebral compression fractures [97]. When a patient, especially one who is elderly, presents with recent onset back pain and demonstrates one or more vertebral compression fractures on radiographs, the clinical question arises whether any (or none) of the fractures is responsible for the pain. Often, all or some of the fractures are old and unrelated to the current pain. Three-phase bone scanning is a reasonably accurate method to date a fracture. Fractures less than about 8 to 10 weeks old generally show abnormal blood pool activity and marked uptake on the delayed images [98]. This is particularly important in the assessment of compression fractures, because kyphoplasty or vertebroplasty is now commonly done with great success in eliminating pain and sometimes spinal deformity but only if performed on acute or subacute fractures [99]. Magnetic resonance imaging usually demonstrates bone marrow edema in recent fractures and is the usual first-line imaging modality for dating compression fractures, with bone scanning usually reserved for patients who cannot undergo MRI because of metallic implants or in whom the MRI was equivocal. Magnetic resonance imaging and CT are being investigated for early diagnosis of a variety of fractures, with generally very good results. For example, a recent study using MRI to detect scaphoid fractures 1 day after injury concluded that this imaging modality may allow for quicker recovery [100].

3. Conclusions Myocardial perfusion imaging is the most exciting, useful, and underused class of NM studies available to emergency physicians, in addition to other NM studies that remain useful in the ED for more carefully selected patient populations. Ventilation/Perfusion scanning is used in patients who cannot receive IV contrast, usually because of poor renal function or prior adverse reaction to contrast. Ventilation/Perfusion scanning is also a good choice in pregnant patients due to low radiation dose to the fetus. High-resolution CT can be used in pregnant patients with suspected PE with an even lower dose to the fetus, but only if the inferior extent of the scan is limited to the xiphoid, which

B. Amini et al. is not standard protocol in the United States. The radiation dose to the maternal breasts is high enough to raise concerns about possible induction of breast cancer, although the cancer risk is quite low and probably acceptable. Finally, there are concerns about the unknown effects of iodinated contrast agents on the fetus. Thus, we feel that a modified V/ Q scan (low dose) is the best imaging study for pregnant patients. Bone scanning remains an accurate test for diagnosing a variety of acute fractures in patients with negative radiography. Computed tomography and MRI, however, are generally as accurate and are usually the firstchoice modality for the reasons discussed above. The most frequently used NM study in the ED is MPI, which has been shown to reduce costs, decrease hospital and intensive care unit stays, and lead to more rapid diagnosis. Unfortunately, MPI remains underused in the ED, largely related to knowledge of its availability, indications, materials, and staffing constraints. Because of the need for immediate information in acute and urgent settings, the use of NM diagnostic procedures must be clearly established for given situations. Limitations to the use of NM techniques include general lack of knowledge about the indications, the fact that it is a purely diagnostic tool, and the degree to which times of day, day of the week, and availability of isotopes limit access to the emergency physician. As with any diagnostic modality, knowledge of the use and pitfalls of NM techniques enhances the diagnostic repertoire of physicians working in acute care settings.

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