Triple Rule-Out CT Scanning In The ED by Stephen Colucciello MD FACEP
Sunday, January 24, 2010 
Stephen Colucciello MD FACEPWHAT IS A TRIPLE RULE-OUT SCAN?
CT angiography (CTA) of the chest can visualize the coronary arteries, pulmonary arteries, and aorta to facilitate the diagnosis of acute coronary syndrome (ACS), pulmonary embolism (PE) and aortic dissection, respectively (the “big three”). In fact, CTA is the test of choice in most hospitals for evaluation of possible pulmonary embolism and aortic dissection. In a minority but growing number of centers, CTA of the coronary arteries is the study of choice to evaluate those at low or moderate risk of ACS.
It is appealing to have a single test that could rule out all “big three” diagnoses in the patient who presents to the ED with chest pain. Unfortunately, the imaging techniques required for optimal visualization of any one of these vascular systems may not be optimal for another. To best evaluate the coronary arteries, ECG-gating (see below) is necessary but this increases scan time and radiation exposure. Thus, the triple rule-out scan is also called an “ECG-gated chest CTA.”
WHAT ARE THE ADVANTAGES OF THE TRIPLE SCAN?
The triple scan can rule out coronary artery disease, pulmonary embolism and aortic dissection with one study. It can also detect pulmonary pathology such as pneumonia and lung cancers; pleural disease such as empyema, and pericardial and myocardial disease; and esophageal disorders. If the scan cuts below the lung bases, it can diagnose abdominal disorders that can cause chest discomfort, such as cholecystitis and pancreatitis. However, the big question remains, “Despite its promise, is the triple rule-out scan just another 'pretty picture'?"
HOW IT IS DONE?
Triple rule-out protocols differ from dedicated coronary CT protocols in that the scanned area is larger (includes the lung apices and bases) and more contrast is necessary to opacify the pulmonary vasculature. The contrast injection is also timed differently. For the triple scan, the injection is prolonged and usually given in a variety of phases (bi- or triphasic). The injection protocol is crucial in obtaining clear images, since the contrast must be synchronized with the CT data acquisition. Computer software then reformats the images into three-dimensional pictures. Triple rule-out protocols deliver more radiation and require an increased breath hold time (up to 20 seconds or more with some scanners). The use of dual-source CT scanners may overcome these limitations to some extent.
WHAT IS ECG GATING AND DOSE MODULATION?
ECG leads are placed on the patient to allow for cardiac gating, which synchronizes the beam to image the heart during diastole, when the heart is briefly motionless. Prospective gating, which is mostly used in calcium scoring (measures calcium in coronary artery plaque), yields a relatively low dosage of radiation (only several mSv) while retrospective gating generates higher dosages (at least 8-12 mSv). Dose modulation (ECG “pulsing”) decreases the tube current during the systolic phase and can reduce total radiation exposure by 30% to 50%.
WHAT TYPE OF SCANNER DO YOU NEED AND HOW LONG DOES THE SCAN TAKE?
Optimal scans are taken during a single breath hold and, in general, the more detectors, the shorter the breath-hold time. While 16-slice scanners can image the coronary arteries, the 64-slice scanner provides dramatically improved temporal resolution. The decreased acquisition time reduces artifact so more patients have high quality images. The 16-slice scan of the entire chest requires nearly 30 seconds and it is difficult to obtain quality images of the aorta, pulmonary arteries, and coronary arteries during a single breath-hold. The 64-slice scan can be done in half this time.
DO YOU NEED TO ADMINISTER ANY MEDICATIONS PRIOR TO THE SCAN?
Beta-blockers are often used to slow the heart rate for patients undergoing gated scans, as artifacts are reduced when the heart rate is less than 65 to 70 beats per minute. Cardiac arrhythmias will also degrade images. Metoprolol 50mg given 1 hour prior to the scan is a reasonable approach, but IV beta-blockers may need to be administered on the CT table, especially if the heart rate remains above 70. The usual contraindications to beta-blockers apply, such as allergy, hypotension, heart block, and asthma.
Nitroglycerin is often given immediately prior to the scan to dilate the coronary arteries and improve visualization. Hypotensive patients, those sensitive to nitrates, and patients taking erectile dysfunction drugs (Viagra, Levitra, and Cialis) should not receive nitrates.
WHAT ARE THE ISSUES SURROUNDING CONTRAST INJECTION?
Contrast timing must be exact to optimize visualization of all three vascular beds (coronary, pulmonary, and aortic). A power injector can provide a triphasic or biphasic bolus that opacifies the important structures and avoids shadowing from the superior vena cava and right heart. This technique yields good visualization of the three vascular beds using a single acquisition technique. The triphasic protocol can also decrease scanning time to 9 seconds.
HOW ACCURATE IS THE TRIPLE SCAN?
In the first study, the authors believe that an ECG-gated CT chest pain protocol provided good visualization of critical thoracic structures in chest pain patients. The scans showed non-coronary pathology including pulmonary embolism, aortic dissection and aortic aneurysm as well as pneumonia, pulmonary edema, pleural effusion, atelectasis and emphysema. The coronary CT angiography had a sensitivity and specificity of 81% and 93% with an average effective radiation dose of 25 mSv.
1. ECG-GATED CHEST CT ANGIOGRAPHY WITH 64-MDCT AND TRI-PHASIC IV CONTRAST ADMINISTRATION REGIMEN IN PATIENTS WITH ACUTE NON-SPECIFIC CHEST PAIN Litmanovitch, D., et al, Eur Radiol 18(2):308, February 2008
METHODS: The authors, from Harvard Medical School, report on their use of "triple CT" scanning in 56 patients with acute chest pain but a normal or inconclusive ECG and negative cardiac enzymes. The imaging consisted of 64-row chest CT angiography using retrospective ECG-gating, ECG pulsing and triphasic IV contrast injection as a comprehensive imaging protocol for assessment of the thorax. Patients with tachypnea, tachycardia, high clinical suspicion for pulmonary embolism, a contraindication to beta-blocker administration, arrhythmias, a pacemaker, compromised renal function or contrast allergy were excluded. RESULTS: The CT studies were felt to be normal in 20 patients (34%), to show vascular non-coronary abnormalities in eleven (20%, including pulmonary embolism, aortic dissection and aortic aneurysm), and to identify a lung or pleural abnormality "consistent with the patient's symptoms" in 20 cases (35%, including pneumonia, pulmonary edema, pleural effusion, atelectasis and emphysema). Coronary CT angiography suggested at least a 50% coronary artery lesion in 16 patients (29%), and (in nine patients who underwent conventional coronary angiography) had a sensitivity and specificity of 81% and 93%. The average effective radiation dose was 25 mSv. CONCLUSIONS: The authors believe this ECG-gated CT chest pain protocol provided good visualization of at least four major vascular and nonvascular components of the chest in patients with acute chest pain. 23 references 6/08 - #1
In the paper by Takakuwa, the authors studied whether a triple scan can safely evaluate emergency department patients with chest pain judged to be at low or moderate risk for ACS. In the 201 patients studied, the CTA showed a noncoronary diagnosis in 11%, suggested significant coronary disease in 11%, and “precluded additional diagnostic cardiac testing in the majority of patients with no adverse outcomes at 30-day follow-up.” Noncoronary diagnoses included pneumonia, pulmonary embolism, cardiomyopathy, congestive heart failure, aortic dissection, cancer, myocarditis, and esophagitis. The triple rule-out scan was able to facilitate safe and rapid discharge of most patients without further testing.
2. EVALUATION OF A "TRIPLE RULE-OUT" CORONARY CT ANGIOGRAPHY PROTOCOL: USE OF 64-SECTION CT IN LOW-TO-MODERATE RISK EMERGENCY DEPARTMENT PATIENTS SUSPECTED OF HAVING ACUTE CORONARY SYNDROME Takakuwa, K.M., et al, Radiology 248(2), August 2008
Purpose: To determine whether coronary computed tomographic (CT) angiography "triple rule-out" evaluation of emergency department (ED) patients presenting with symptoms suggestive of acute coronary syndrome (ACS) can help identify a subset of patients who can be discharged without adverse clinical outcomes within 30 days. Materials and Methods: This protocol was approved by the university institutional review board. Each patient provided written informed consent prior to inclusion. Coronary CT angiography was performed in 201 consecutive low-to-moderate risk ACS patients. A triple rule-out protocol was used to evaluate for coronary disease, pulmonary embolism, aortic dissection, and other thoracic disease. Four patients were excluded because of technical problems. The remaining subjects underwent a 30-day follow-up. Results: A disease process other than coronary atherosclerosis that explained the presenting symptoms was diagnosed in 22 (11%) of 197 patients. Clinically important noncoronary diagnoses that did not explain patient symptoms were identified in 27 (14%) of 197 additional patients. With respect to coronary artery disease, 10 patients had severe disease (>70% stenosis), 12 had moderate disease (50%–70% stenosis), 46 had mild disease (up to 50% stenosis), and 129 had no disease. No further diagnostic testing was performed in 133 (76%) of 175 of patients with no to mild coronary disease. At 30-day follow-up, the negative predictive value of coronary CT angiography with no more than mild disease was 99.4%. There were no adverse outcomes at 30 days. Conclusion: Triple rule-out coronary CT angiography evaluation of low-to-moderate risk ACS patients presenting to the ED provided a noncoronary diagnosis that explained the presenting complaint in 11% of patients, suggested the presence of significant moderate-to-severe coronary disease in 11% (22 of 197) of patients, and precluded additional diagnostic cardiac testing in the majority of patients with no adverse outcomes at 30-day follow-up.
CAN WE ADEQUATELY EVALUATE THE PULMONARY ARTERIES AND THE AORTA USING A CARDIAC CT ANGIOGRAPHY PROTOCOL?
Probably not. Excellent images of the pulmonary arteries and aorta occur at the expense of lower quality images of the coronary arteries. Patients undergoing a triple rule-out scan require special contrast injection and a larger dye load.
In a retrospective examination of 50 patients imaged with a 64-slice CTA, the authors studied “whether a dedicated coronary CT protocol provides adequate contrast enhancement and artifact-free depiction of coronary, pulmonary, and aortic circulation.” They found that the dedicated coronary CT protocol provided excellent visualization of the coronary arteries and proximal ascending aorta “but does not depict the pulmonary vasculature well enough for exclusion of pulmonary embolism.”
3. EMERGENCY CARDIAC CT FOR SUSPECTED ACUTE CORONARY SYNDROME: QUALITATIVE AND QUANTITATIVE ASSESSMENT OF CORONARY, PULMONARY, AND AORTIC IMAGE QUALITY Dodd, J.D., et al, Am J Roent 191:870, September 2008
OBJECTIVE: The purpose of this study was to determine whether a dedicated coronary CT protocol provides adequate contrast enhancement and artifact-free depiction of coronary, pulmonary, and aortic circulation. MATERIALS AND METHODS: Dedicated coronary 64-MDCT data sets of 50 patients (27 men; mean age, 54 ± 12.4 years) consecutively admitted from the emergency department with suspected acute coronary syndrome were analyzed. Two independent observers graded overall coronary arterial image quality and qualitative and quantitative contrast opacification, motion, and streak artifacts within the pulmonary arteries and aorta. RESULTS: Coronary image quality was excellent in 48 patients (96%) and moderate in two patients (4%). Eleven left main and 22 left upper lobar pulmonary arteries were not visualized. Qualitative evaluation showed pulmonary arterial tree opacification to be excellent except for the right and left lower lateral and posterior segmental branches (52–54% rate of poor opacification). Quantitative evaluation showed four central (8%), six lobar (8%), and 206 segmental (29%) branches had poor contrast opacification (< 200 HU). Nineteen right upper lobar arteries (38%) were slightly and one was severely affected by streak artifact. At the segmental pulmonary artery level, marked differences in contrast enhancement were detected between the upper (292 ± 72 HU) and both the middle (249 ± 85 HU) and the lower lobes (248 ± 76 HU) (p < 0.01). Mean aortic opacification was 300 ± 34 HU with excellent contrast homogeneity without severe motion or streak artifacts. CONCLUSION: In the evaluation of patients presenting to the emergency department with suspected acute coronary syndrome, a dedicated coronary CT protocol enables excellent assessment of the coronary arteries and proximal ascending aorta but does not depict the pulmonary vasculature well enough for exclusion of pulmonary embolism.
WHAT PATIENT POPULATIONS ARE NOT WELL SUITED FOR THE TRIPLE SCAN?
Coronary calcifications, cardiac stents, bypass graft clips may all produce artifacts on the MDCT scans. Patients with arrhythmias, tachycardia, and tachypnea, those unable hold still or hold their breath may have suboptimal scans. Patients with renal insufficiency are at higher risk for contrast-induced nephropathy from the high dye load associated with the triple rule out scan. Elevated creatinine levels may be a contraindication for the protocol. Contrast allergy, depending upon the severity, may also prohibit CTA.
WHAT ARE THE TECHNICAL LIMITATIONS OF THE TRIPLE SCAN?
While a number of studies show that the MDCT scan is useful in ruling out coronary artery disease (CAD) in low- to moderate-risk patients, other studies show a disturbing number of false-positive and false-negative scans. In addition, MDCT is often unable to evaluate the exact degree of stenosis in those diagnosed with coronary lesions. In the below multinational trial of nine centers and 291 patients, researchers found that CTA failed to detect significant coronary lesions in 15% of patients, and incorrectly diagnosed blockages that were not present in 10% of cases. In this study, published in the New England Journal of Medicine, the diagnostic accuracy of quantitative CT angiography for detecting or ruling out stenoses of 50% or more compared to conventional angiography revealed a sensitivity of 85%, a specificity of 90%, a positive predictive value of 91%, and a negative predictive value of 83%. The authors state that “multi-detector CT angiography cannot replace conventional coronary angiography at present.”
4. DIAGNOSTIC PERFORMANCE OF CORONARY ANGIOGRAPHY BY 64-ROW CT Miller, J.M., et al, NEJM 359;22 November 27, 2008
BACKGROUND: The accuracy of multidetector computed tomographic (CT) angiography involving 64 detectors has not been well established. METHODS: We conducted a multicenter study to examine the accuracy of 64-row, 0.5-mm multidetector CT angiography as compared with conventional coronary angiography in patients with suspected coronary artery disease. Nine centers enrolled patients who underwent calcium scoring and multidetector CT angiography before conventional coronary angiography. In 291 patients with calcium scores of 600 or less, segments 1.5 mm or more in diameter were analyzed by means of CT and conventional angiography at independent core laboratories. Stenoses of 50% or more were considered obstructive. The area under the receiver-operating-characteristic curve (AUC) was used to evaluate diagnostic accuracy relative to that of conventional angiography and subsequent revascularization status, whereas disease severity was assessed with the use of the modified Duke Coronary Artery Disease Index. RESULTS:A total of 56% of patients had obstructive coronary artery disease. The patient-based diagnostic accuracy of quantitative CT angiography for detecting or ruling out stenoses of 50% or more according to conventional angiography revealed an AUC of 0.93 (95% confidence interval [CI], 0.90 to 0.96), with a sensitivity of 85% (95% CI, 79 to 90), a specificity of 90% (95% CI, 83 to 94), a positive predictive value of 91% (95% CI, 86 to 95), and a negative predictive value of 83% (95% CI, 75 to 89). CT angiography was similar to conventional angiography in its ability to identify patients who subsequently underwent revascularization: the AUC was 0.84 (95% CI, 0.79 to 0.88) for multidetector CT angiography and 0.82 (95% CI, 0.77 to 0.86) for conventional angiography. A per-vessel analysis of 866 vessels yielded an AUC of 0.91 (95% CI, 0.88 to 0.93). Disease severity ascertained by CT and conventional angiography was well correlated (r=0.81; 95% CI, 0.76 to 0.84). Two patients had important reactions to contrast medium after CT angiography. CONCLUSIONS: Multidetector CT angiography accurately identifies the presence and severity of obstructive coronary artery disease and subsequent revascularization in symptomatic patients. The negative and positive predictive values indicate that multidetector CT angiography cannot replace conventional coronary angiography at present. (ClinicalTrials.gov number, NCT00738218
WHAT ARE THE RISKS OF THE TRIPLE SCAN?
Over-testing! The greatest risk of the triple-rule out scan is that it is so easy and appealing for the physician to order. Some careless physicians might see the test as a way of simplifying their life at the expense of patient care; get rid of detailed history and physical exam, no need for serial markers, no more d-dimers, no more chest x-ray, no more overnight stays in a Chest Pain Observation unit. One check box and voila, chest pain workup complete! While it may be safe for the physician in terms of lowering malpractice risk (“Hey, I checked for EVERYTHING” in this patient!) what is the ultimate cost to the patient in terms of future cancer risk? To the millions of chest pain patients that come to the ED each year?
RADIATION RISK
Radiation exposure with triple-rule out CT angiography (up to 24 mSv) equals approximately 250 chest x-rays. This is significantly greater than conventional coronary angiography (approximately 8 mSv) and is more than double that of a dedicated CTA of the chest using a rule-out PE protocol. The gated technique may generate twice the radiation dosage of the non-gated CT angiogram (25 mSv versus 12 mSv in one study by Litmanovitch).
CONTRAST LOAD
Because multiple vascular structures require opacification, more contrast is needed for the triple scan than for any single protocol scan looking solely for ACS, PE, or dissection.
WHO IS AT GREATEST RISK FROM RADIATION?
Radiation risk from CTA varies with age, sex, scan protocol and scan technology. In the below JAMA study, the authors examined the lifetime attributable risk (LAR) of cancer incidence associated with radiation exposure from CTA studies. They showed that the “lifetime cancer risk estimates for standard cardiac scans varied from 1 in 143 for a 20-year-old woman to 1 in 3261 for an 80-year-old man. Use of simulated electrocardiographically controlled tube current modulation (ECTCM) decreased these risk estimates to 1 in 219 and 1 in 5017, respectively.” One important question is, if the risk of cancer in a young woman is 1 in 200 for a single scan, do four scans or their equivalent over several years increase that risk to 1 out of 50? Is the cumulative risk geometric or worse?
5. ESTIMATING RISK OF CANCER ASSOCIATED WITH RADIATION EXPOSURE FROM 64-SLICE COMPUTED TOMOGRAPHY CORONARY ANGIOGRAPHY Einstein, A.J., et al, JAMA 298(3):317, July 18, 2007
BACKGROUND: It is generally perceived that radiation exposure associated with 64-slice CT coronary angiography (CTCA) increases the risk of cancer, but age- and gender-specific risk has not been clearly defined. METHODS: The authors, from Columbia University and Mt. Sinai Medical Center in New York and Ohio State University, estimated the lifetime attributable risk (LAR) of cancer, based on the Biological Effects of Ionizing Radiation (BEIR) VII report, associated with a single CTCA study. Three CTCA protocols were considered: a standard cardiac CTCA study, an EKG-based dose reduction strategy, and a scanning strategy that extends cranially to include the aortic arch for "triple rule out" purposes (CAD, aortic dissection and pulmonary embolism). RESULTS: Among women, the LAR for cancer with a standard CTCA study is 0.7% (1 in 143) when CTCA is performed at age 20, and 0.35% (1 in 284) with CTCA at age 40, 0.22% (1 in 466) at age 60 and 0.075% (1 in 1,338) at age 80, with lung and breast cancer accounting for 80-85% of such cancers at all ages. Among men, the corresponding LARs at age 20, 40, 60 and 80 are 0.15% (1 in 686), 0.099% (1 in 1,007), 0.081% (1 in 1,241) and 0.044% (1 in 3,261, respectively. The cancer risk is reduced by about 35% with a CTCA EKG-based dose-reduction strategy, and increased by about 24-28% in women and 43-46% in men with a "triple rule out" cranial extension approach. CONCLUSIONS: The lifetime attributable risk of cancer associated with 64-slice CTCA is not negligible, particularly for younger patients. 29 references 1/08 - #40
Patients with a history of multiple prior high-radiation imaging studies are at greater risk for future radiation-induced cancers. As importantly, any CTA we order on a patient today is unlikely to be their last. In a study by Kline, 675 patients were followed for approximately 2000 days. Seventy three percent had one or more subsequent CT scans of any body part and 5% had five or more repeated CT pulmonary angiograms. At least one-third of ED patients who undergo CT pulmonary angiography scanning will have a second CT pulmonary angiography result that will be negative for pulmonary embolism.
6. INCIDENCE AND PREDICTORS OF REPEATED COMPUTED TOMOGRAPHIC PULMONARY ANGIOGRAPHY IN EMERGENCY DEPARTMENT PATIENTS Kline J.A., Ann Emerg Med, October 2008
Study Objective: Use of contrast-enhanced computed tomography (CT) of the pulmonary arteries to evaluate for pulmonary embolism has increased, raising concern about radiation and contrast toxicity. We sought to measure the frequency of repeat CT pulmonary angiography in emergency department (ED) patients. Methods: This was a prospective, longitudinal follow-up of ED patients who underwent first-time CT pulmonary angiography as part of a research protocol for diagnosis of pulmonary embolism in 2001 to 2002. Two authors (DMB and MCK) searched electronic medical record databases to measure the frequency of repeated CT scans performed within 5 years. Primary outcome was greater than or equal to 1 repeated CT pulmonary angiography examination. Radiologist-written interpretations of CT pulmonary angiography were categorized by 2 observers (DMB and JAK). Cox regression was used to estimate hazard ratios for 24 clinical variables. Results: A cohort of 675 ED patients was observed for a median of 1,989 days: 226 of 675 (33%) had at least 1 additional CT pulmonary angiography scan, and 60 died with no repeated CT pulmonary angiography, leading to a mortality-adjusted frequency of repeated CT pulmonary angiography scanning of 226 of 615, or 37%. Seventy-three percent of the cohort had 1 or more subsequent CT scans of any body part, and 31 patients (5%) had 5 or more repeated CT pulmonary angiography scans. The pulmonary embolism (positive) prevalence was 57 of 675 (8.4%; 95% confidence interval [CI] 6.5% to 10.8%) on the first CT pulmonary angiography versus 8 of 226 (3.5%; 95% CI 1.5% to 6.9%) on the second CT pulmonary angiography scan. Hazard ratios indicated that respiratory rate, active malignancy, previous coronary artery disease, and previous or new diagnosis of venous thromboembolism were positively associated with repeated CT pulmonary angiography scanning. Conclusion: At least one third of ED patients who undergo CT pulmonary angiography scanning will have a second CT pulmonary angiography result that will be negative for pulmonary embolism. New methods are needed to exclude pulmonary embolism recurrence without use of ionizing radiation.
WHAT ALTERNATIVE STRATEGIES EXIST TO REDUCE RADIATION DOSAGE?
V/Q SCANNING
The radiation dose to the breast is about 0.28-0.9 mSv with V/Q scanning, but 20-60 mSv with 4-slice CT and 50-80 mSv with 64-slice CT. It has been estimated that the lifetime attributable risk of cancer associated with CT angiography (CTA) is 1 in 143 for a 20-year-old woman and 1 in 248 for a 40-year-old woman. In the following report, the author reduced radiation exposure by using plain chest x-rays to determine the most appropriate imaging modality for suspected PE. It is suggested that an abnormal chest x-ray warrants CTA while a normal chest x-ray warrants a V/Q scan, a strategy that has been associated with a 27% decrease in the use of CTA and an 82% increase in V/Q scanning at his institution. However, patients with a low-probability VQ scan and high clinical suspicion, and all patients with an indeterminate or moderate probability scan will need additional imaging beyond that of the original VQ scan, thus increasing their individual risk of radiation exposure.
7. DON'T BURY THE V/Q SCAN: IT'S AS GOOD AS MULTIDETECTOR CT ANGIOGRAMS WITH A LOT LESS RADIATION EXPOSURE Freeman, L.M., J Nucl Med 49(1):5, January 2008
The author, from Montefiore Medical Center in New York, comments on the hazards associated with the tremendous increase in the use of CT scanning for possible pulmonary embolism (PE), which is likely, at least partially, fueled by a misguided belief that it is more accurate than V/Q scanning in addition to economic considerations. The radiation dose to the breast is about 0.28-0.9 mSv with V/Q scanning, but 20-60 mSv with 4-slice CT and 50-80 mSv with 64-slice CT. It has been estimated that the lifetime attributable risk of cancer associated with CT angiography (CTA) is 1 in 143 for a 20- year-old woman and 1 in 248 for a 40-year-old woman. The sensitivity and specificity of CTA for PE in PIOPED II were 83% and 96%, respectively. Similarly, the likelihood of PE exceeds 85% in patients with high-probability V/Q scans and is less than 20% with low-probability V/Q scans. Although no diagnostic imaging is necessary in some cases (e.g., the patient with a low-probability Wells score and a negative D-dimer), physicians often order a confirmatory CT scan to relieve their own diagnostic discomfort. This author has found that plain chest x-rays are a reliable guide for ascertaining the most appropriate imaging modality, suggesting that an abnormal chest x-ray warrants CTA while a normal chest x-ray warrants a V/Q scan, a strategy that has been associated with a 27% decrease in the use of CTA and an 82% increase in V/Q scanning at his institution. As patient advocates, physicians are advised to be aware of the high level of radiation exposure associated with CTA for pulmonary embolism and to carefully consider the risks and benefits of available diagnostic imaging modalities for such patients. 26 references 6/08 - #40
LIMIT CANDIDATES FOR THE TRIPLE SCAN
The most compelling strategy to reduce radiation exposure is to limit the population eligible for the protocol. For instance, some centers do not perform the triple scan on patients under the age of 50 as the incidence of ACS is lower and the risk of radiation-related cancer much higher. The use of d-dimer testing may also decrease the need for CT angiography. A negative d-dimer will rule out PE in the patient with a low pretest probability for the disease. In some studies, all or nearly all patients with aortic dissection have an abnormal d-dimer.
NEW TECHNOLOGY
New technology includes dual source MDCT (Siemens Medical Solutions, Malvern, PA) and the upcoming 256-row non-spiral MDCT (Toshiba Medical Systems, Tokyo, Japan) with prospective ECG gating. These technical advances may decrease scan time and radiation load.
WHAT DO THE NORTH AMERICAN SOCIETY OF CARDIAC IMAGING AND THE EUROPEAN SOCIETY OF CARDIAC RADIOLOGY SAY ABOUT THE ROLE OF MDCTA IN CHEST PAIN EVALUATION?
In this consensus statement, the authors note that the exact role of MDCT for the identification or exclusion of ACS in the ED requires further study. They consider MDCT to be the diagnostic test of choice for pulmonary embolism and acute aortic syndromes, but admit there is no consensus regarding the single MDCT protocol for a "triple rule-out" study to include coronary assessment.
8. USE OF MULTIDETECTOR COMPUTED TOMOGRAPHY FOR THE ASSESSMENT OF ACUTE CHEST PAIN: A CONSENSUS STATEMENT OF THE NORTH AMERICAN SOCIETY OF CARDIAC IMAGING AND THE EUROPEAN SOCIETY OF CARDIAC RADIOLOGY Stillman, A.E., et al, Eur Radiol 17(8):2196, August 2007
BACKGROUND: In the U.S., the hospital admission rate for patients presenting with chest pain is about 30-70%, but an acute coronary syndrome (ACS) is ultimately diagnosed in only 15-25%. Conversely, ACS is subsequently identified in 2-8% of chest pain patients discharged from the ED, a population at risk for increased morbidity and mortality. Multidetector CT scanning (MDCT) has been reported to be very accurate for the detection of coronary heart disease, but there are concerns about widespread application of this technology without definitive supporting evidence. METHODS: These multinational authors, from the North American Society for Cardiac Imaging (NASCI) and the European Society of Cardiac Radiology (ESCR), provide interim guidance regarding the use of MDCT in patients presenting to the ED with chest pain. RESULTS: Rational use of MDCT when ACS is a possibility is dependent upon risk stratification. According to these authors, MDCT is likely to be most beneficial when the ED evaluation for ACS is inconclusive. They point out, however, that the exact role of MDCT for the identification or exclusion of ACS in the ED, including patient populations most likely to benefit, requires clarification. They do consider MDCT to be the diagnostic test of choice for pulmonary embolism and acute aortic syndromes, but point out that that there is no consensus regarding the single MDCT protocol that would accurately, expeditiously and cost-effectively serve as a "triple rule out" study. 52 references 1/08 - #4
KEY POINTS AND RECOMMENDATIONS
1. CT angiography (CTA) of the chest can visualize the coronary arteries, pulmonary arteries, and aorta to facilitate the diagnosis of acute coronary syndrome (ACS), pulmonary embolism (PE), and aortic dissection respectively (the “big three”).
2. Besides detecting coronary artery disease, pulmonary embolism and aortic dissection, the triple scan can also detect pulmonary pathology such as pneumonia and lung cancers; pleural disease such as empyema, and pericardial and myocardial disease; and esophageal disorders. If the scan cuts below the lung bases, it can diagnose abdominal disorders that cause chest discomfort, such as cholecystitis and pancreatitis.
3. To obtain optimal accuracy, use protocols that involve 64-slice machines with ECG gating and multiphase injection protocols. Beta-blockers and nitroglycerin administered before the scan can improve visualization of the coronary arteries.
4. Radiation risk is the greatest concern with the triple rule-out scan. Don’t fry the young. Consider NOT doing a triple rule-out in patients younger than 40 (especially in younger women who are at the highest risk for cancer).
5. Use scoring systems for PE. If PE is suspected, use a scoring system to decrease the number of CT scans ordered. The PERC rule identifies patients with such a low risk of PE that a d-dimer is not necessary (thus eliminates the unnecessary CTA because of a false-positive d-dimer).
6. Use d-dimers appropriately. A negative d-dimer will essentially rule out PE and aortic dissection in patients with a low risk of disease. Some argue that a normal d-dimer is almost never seen with aortic dissection.
7. Develop a clinical score. A clinical score or likelihood estimate of PE, ACS, and aortic dissection can help focus the diagnostic approach. If ACS is the most reasonable diagnosis and PE and aortic dissection are relatively unlikely, perform a targeted workup for ACS using ECG, cardiac markers and stress testing/cardiac imaging.
8. Check the computer or medical records. Before ordering a CTA for chest pain, see what prior work-ups have been done. Has the patient had a cardiac cath in the past year? A high quality stress test in the past 6 months? A recent negative work-up may decrease the need for a CTA. Also check to see how many CT scans the patient has received in the past several years. A significant percentage of patients who undergo CTA get multiple scans.
9. Consider alternatives to CTA. While CTA is the study of choice for patients suspected of having aortic dissection. other alternatives exist to evaluate for ACS and PE. In most hospitals, ECGs, serial markers, stress testing and/or cardiac imaging remain the standard protocol. Because of the lower radiation exposure to breast tissue, V/Q scans are an excellent alternative to CTA in the young woman suspected of PE who has a normal chest x-ray.
10. Develop local consensus. Involve cardiology, radiology, emergency medicine, and community physicians in protocol development. To achieve optimal results, you will need a 64-slice MDCT with ECG gating and a computerized power injector protocol (bi-or triphasic). Strict exclusion and inclusion criteria will be necessary to avoid sub-optimal scans and unnecessary radiation of patients in whom MDCT scans may not be warranted.
11. Stay tuned. The advent of the 256-slice scan, multi-source scanners, radiation reduction protocols, and new computerized programs could rapidly change the odds in favor of triple rule-out scans. However, we must continue to base our work-up upon data provided by large well-designed studies as opposed to diagnostic convenience or “gee-whiz” images.
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