7.15
Cardiothoracic Multi-Slice CT in the Emergency Department U.-J. Schöpf
select cases, in which critical therapeutic decisions (i.e., anticoagulation in high-risk patients) hinge on the diagnosis or exclusion of pulmonary embo- lism, ECG gating may be used to improve diagnostic quality at pulmonary CTA to differentiate between artifactual filling defects in the pulmonary arteries, caused by cardiac pulsation, and real thromboem- bolic clots (Fig. 7.110). ECG gating may also refine the derivation of functional parameters from con- trast-enhanced CTA, which provides an important tool for risk stratification in patients with acute pul- monary embolism (Schöpf 2004b).
However, the through-plane spatial resolution that can be achieved with retrospectively ECG-gated technique using 4- or 8-slice multi-slice CT scanners is limited by the relatively long scan duration inher- ent to data oversampling. Thus, high-resolution acquisition can only be achieved for relatively small volumes, i.e., the coronary arterial tree, but not for extended coverage of the entire chest. The advent of 16-slice CT scanners enabled a substantial volume of the body to be scanned with retrospective ECG gating and high through-plane resolution in a single breath-hold (Flohr 2003). This capability allows non-invasive assessment of systemic manifestations of atherosclerotic disease throughout the body with sufficient vascular detail to evaluate the coronary arteries based on a single, contrast-enhanced scan (Fig. 7.111).
Although 16-, 32-, and 40-slice CT scanners enable comprehensive assessment of the entire arte- rial system for atherosclerosis and other vascular disease, they are still limited by several obstacles.
For example, the current state-of-the-art for coro- nary CTA for non-invasive assessment of coronary artery disease comprises acquisition of sub-milli- meter sections (Flohr 2003), but this is difficult to achieve for large anatomic volumes. In addition, the robustness of previous multi-slice CT generations with respect to faster and more irregular heart rates still requires the use of β-blockers for rate control in a substantial percentage of patients. Breath-hold times are still relatively long, which poses a particu- lar difficulty if this technique is to be used to evalu- ate critically ill patients, whose ability to cooperate during scan acquisition is limited. Similarly, long scan times prohibit simultaneous assessment of the pulmonary arterial and systemic arterial circulation
C o n t e n t s
7.15.1 ECG-Gated Multi-Slice CT Scanning of the
Chest 317
7.15.2 Patients with Equivocal Chest Pain in the Emergency Department 320
7.15.3 64-Slice CT as a Triage Tool in the Emergency Department 320
References 324
7.15.1
ECG-Gated Multi-Slice CT Scanning of the Chest Cardiac motion artifacts degrade the diagnostic quality of thoracic CT. Some of these artifacts are recognized as an important source of potential diagnostic error (Loubeyre 1997). Synchroniza- tion of the CT scan acquisition with the patient’s ECG reduces cardiac motion artifacts and enables non-invasive visualization of the coronary arteries (Ohnesorge 2000, Schöpf 2004a) and other cardiac anatomy.
ECG synchronization has also been shown to improve image quality at CT imaging of non-cardiac thoracic structures (Schöpf 1999, Hofmann 2004).
ECG-synchronized acquisition during high-resolu- tion CT of the lung eliminates artifacts attributable to cardiac motion (Fig. 7.108) and thus improves the diagnosis of diffuse lung disease. In many insti- tutions, retrospective ECG gating has become an integral component in the imaging protocol of sus- pected disease of the ascending aorta. The use of ECG gating avoids pulsation and doubling artifacts of the aortic root and the ascending aorta (Flohr 2002) (Fig. 7.109), which often are misinterpreted by less-experienced observers as aortic dissection and may result in unnecessary emergency surgery. In
Fig. 7.109a–d. A 54-year-old man with a mean heart rate of 62 bpm, examined with 4-slice CT. Double-oblique multi-planar reformats of the aortic root at the level of the aortic valve as seen from a cranial perspective. Image reconstruction was refer- enced to four different time points during the cardiac cycle. Motion artifacts during systolic opening of the valve at 0% RR (a) and 20% RR (b) preclude clear demarcation of the valvular cusps. Delineation of the cusps is best at mid-diastolic closure at 60% RR (c) and slightly deteriorates again during late diastole at 80% RR (d)
a b
c d
Fig. 7.108a,b. Non-ECG triggered (a) and ECG-triggered (b) high-resolution CT scans obtained with 4-slice CT at the same level of the paracardiac parenchyma in a 24-year-old man who underwent lung transplantation. Mild dilatation and bronchial wall thickening in the left lower lobe (white arrows) are better seen in the ECG-triggered study than in the conventional scan. Also, note blurring of the cardiac border (black arrow) in the non-ECG triggered scan
a b
Fig. 7.110a,b. Contrast-enhanced multi-slice CT angiography using 4-slice CT in a 60-year-old woman with a mean heart rate of 73 bpm during scan acquisition. Focused, colored volume-rendered display of the paracardiac pulmonary circulation seen from a left lateral perspective. Image reconstruction with use of 0% RR (a) and 80% RR (b) as respective starting points. Im- age reconstruction during late diastole (b) results in a reduction of stair-stepping artifacts (arrows), which cause artifactual discontinuity of vessels during systole (a). A small subsegmental branch (arrow) of a pulmonary artery that supplies the lingula of the left upper lobe of the lung is only visualized at image reconstruction during late diastole (b)
a b
Fig. 7.111a–c. Comprehensive systemic assessment of cardiovas- cular disease. A 55-year-old male with epigastric pain was exam- ined using 16-slice CT angiogra- phy with a gantry rotation time of 420 ms and retrospective ECG gating. The CT scan demonstrates a circumscribed type B dissection of the descending thoracic aorta originating distal to the origin of the left subclavian artery (a, b). In addition, extensive atherosclerotic changes with heavy calcifi cations are noted in the wall of the ab- dominal aorta (b). High-resolu- tion image acquisition and the absence of cardiac pulsation arti- facts owing to retrospective ECG synchronization enables assess- ment of the coronary artery tree from the same scan (c) and shows extensive atherosclerotic changes of the RCA
RCA D
b
a c
in a single, high-resolution ECG-gated scan, as these scans would require unreasonably high amounts of contrast material to maintain enhancement of both vascular systems throughout the scan. These limita- tions prevented application of the previous genera- tion of multi-slice CT scanners to achieve another coveted goal: comprehensive assessment of patients with acute chest pain, typically in an emergency department setting.
7.15.2
Patients with Equivocal Chest Pain in the Emergency Department
Every year, over 1.5 million Americans are admit- ted to the hospital after presenting to the emergency department with acute chest pain. Only a small per- centage of those admitted have coronary syndromes, whereas the vast majority of patients have non-car- diac diagnoses. In many patients, the findings on ini- tial evaluation are equivocal or indeterminate. Rapid diagnostic assessment of the causes of the acute chest pain would provide a suitable hospital-based test that could substantially reduce mortality and treatment costs (Lee 1987, Yusuf 1988, Oler 1996).
Several strategies have emerged to manage these patients. Patients with typical features of unstable angina or acute myocardial infarction, such as ECG changes typical of ischemia, or those with known CHD and prolonged cardiac-type pain are at sub- stantial risk of adverse events and hospital admis- sion is mandatory (Pozen 1984, Panju 1998). On the opposite end of the spectrum, patients with no risk factors for CHD, no clinical features of cardiac chest pain, and a normal ECG are usually discharged home. A diagnostic dilemma, however, exists for the substantial number of patients who present with acute chest pain with an intermediate probability of having CHD and a normal or non-diagnostic ECG.
Recently, new cardiac enzyme tests have improved early sensitivity and specificity for detecting acute myocardial infarction. However, these tests do not achieve acceptable levels of sensitivity until at least 6 h after the onset of pain. As patients typically present earlier than this, a period of observation is inevitably combined with use of such tests, which prolongs hos- pital stay. In order to improve the sensitivity for diag-
nosis of unstable angina, provocative cardiac testing (typically exercise treadmill) has been used in combi- nation with monitoring, enzyme testing, and provoc- ative testing. This strategy has become widespread in the USA, usually in a designated chest pain observa- tion unit (Zalenski 1998). Although this approach is more sensitive than enzyme testing alone, it exerts a significant cost burden on the health-care system.
Current strategies therefore range from low-cost, low-benefit (discharge home) to high-cost, high-ben- efit (hospital admission). Data relating to these strat- egies are emerging but remain largely anecdotal. Yet, the potential burden in terms of morbidity, mortality, and socioeconomic resources demands a coherent and rational approach.
The common way of ruling out coronary artery disease as the underlying cause is coronary cath- eterization, which consequently is done in many patients who have chest pain but no typical clini- cal presentation for coronary artery stenosis or blockage. Although the presence of coronary artery disease is unlikely, these patients usually undergo this invasive procedure just to rule out the remote possibility that significant coronary artery disease is causing their symptoms. With invasive coronary angiography, contrast material is directly injected into the coronary arteries, thus allowing their visu- alization as a cast-like fluoroscopic X-ray image.
Unfortunately, coronary catheterization is expen- sive and invasive and the risk to the patient is not negligible. Also, coronary angiography is not capa- ble of identifying other causes of chest pain, such as aortic dissection or pulmonary embolism. Thus, additional imaging is frequently necessary, increas- ing the length of stay, costs, and radiation exposure to the patient. The ability to differentiate cardiac and non-cardiac causes of acute chest pain without the need for such an invasive procedure would greatly benefit affected individuals and decrease the costs for a comprehensive diagnostic work-up.
7.15.3
64-Slice CT as a Triage Tool in the Emergency Department
With the advent of 64-slice CT, a tool is now available to effectively eliminate previous limitations of CT
and enable the rapid triage of patients with equivo- cal chest pain. Since 64-slice CT provides isotropic spatial resolution of less than 0.4 mm, visualization of small vascular detail is greatly enhanced. Gantry rotation times of 330 ms significantly improve the temporal resolution of cardiac CT scan acquisi- tions to a constant 165 ms at all heart rates. This improved temporal resolution directly translates into substantially enhanced robustness of cardiac scan acquisition at higher and more irregular heart rates. In turn, this reduces the need for rate con- trol and significantly increases the percentage of patients in whom a fully diagnostic scan can be obtained. The need for using artifact-prone multi- segment reconstruction algorithms for improving temporal resolution is reduced. Breath-hold times are significantly decreased. With accurate timing of the contrast bolus, the rapid scan acquisition enables ECG-gated interrogation of both the pul- monary arterial and systemic arterial circulation of the chest with a single injection of a standard bolus of contrast material (Fig. 7.112). The combina- tion of these achievements should fulfill the tech- nical prerequisites for integrating 64-slice CT into the diagnostic algorithm of patients with equivocal chest pain.
The technical obstacles for the clinical application of this test are being overcome; thus, the appropri- ate indication of this test within the work-up of chest pain patients still needs to be carefully defined. Cer-
tainly, patients who have a very low pre-test likeli- hood of coronary artery disease and whose benignity of symptoms would warrant speedy discharge from the emergency department are unlikely to benefit from this test. However, patients with a very high pre-test likelihood of coronary artery disease as a source of their chest pain (e.g., known CHD, ST eleva- tion, positive cardiac markers) will undergo conven- tional work-up including catheterization anyhow.
For such patients, the use of 64-slice CT as an addi- tional diagnostic test will likely not be beneficial but rather detrimental, as the onset of specific therapy (e.g., thrombolysis, stent placement) may be delayed and complicated by an additional load of iodine, which may impede lengthy catheter interventions.
Instead, 64-slice CT seems exceedingly well-suited for quickly and non-invasively triaging patients with equivocal presentation, non-diagnostic ECG, and initially negative serum markers for acute myo- cardial injury (Fig. 7.113). As described above, such patients ordinarily undergo a period of observation with serial assessment of ECG and cardiac markers and eventually further work-up, such as stress test- ing (Braunwald 2002). In these patients, the inclu- sion of 64-slice CT into the diagnostic algorithm would enable rapid diagnosis of common non-car- diac causes of acute chest pain, such as acute pulmo- nary embolism (Schöpf 2004c), aortic dissection, or aortic and pulmonary aneurism, each of which is dif- ficult to diagnose with the tools and tests comprising the conventional work-up of patients with chest pain (Fig. 7.114). Similarly, important additional diagno- ses that impact on patient management in the pres- ence or absence of a specific reason for chest pain are very amenable to CT visualization (Fig. 7.115). There is agreement that negative coronary CTA is associ- ated with a very high negative predictive value for the exclusion of significant coronary artery stenosis (Schoepf 2004a). Thus, significant coronary artery disease as a source of symptoms in patients with acute chest pain should be safely ruled out by a nega- tive CT coronary angiogram (Fig. 7.116) of sufficient diagnostic quality. If significant coronary artery disease can be specifically diagnosed by the contrast enhanced 64-slice CT scan (Fig. 7.117), treatment can be initiated. There will still be patients in whom the CT scan will not be fully diagnostic or explanatory for their symptoms. Those patients will still have to
Fig. 7.112. Sample contrast-media injection protocol for 64- slice CT of the chest. A bolus of 120 ml of contrast material, injected at 4 ml/s enables high-resolution (i.e., sub-millimeter) ECG-gated image acquisition during a 20-s overlap phase, with contrast enhancement of the systemic and PA side of the tho- racic vasculature
120 cc CM @ 4 cc/s = 30 s
50 cc NaCl
64 SLICE SCAN
0 10 20 30 40 50 60
20 s breathhold
PA enhancement
Syst. arterial enhancement
Acute chest pain
No CT
CT
Non-cardiac (Pulmonary embolism
Aortic dissection Pericardial eff usion etc.)
Possible acute coronary syndrome
Likely acute coronary syndrome No ST
elevation
ST elevation
Positive markers Non-diagnostic ECG
Normal markers
Repeat ECG Repeat markers No recurrent pain
Negative markers
Stress test Further workup
Recurrent pain Positive markers
Appropriate therapy
Fig. 7.113. Coronary CT angiography in the clinical work-up of patients with acute chest pain in the emergency department. CT investigation of patients with a high likelihood (ST-elevation and/or positive cardiac markers) of having an acute coronary syndrome will not benefi t
Fig. 7.114a,b. ECG-gated 64-slice CT examination of a patient with chest pain. A scan range of 25 cm was covered in an 18 s breath-hold time. A pulmonary aneurysm is evident in sagittal MPR (a) and VRT reconstruction (b). MPR reconstruction also reveals a calcifi ed lesion in the left main coronary artery but no related coronary stenosis. (Case courtesy of Mayo Clinic Rochester)
b a
Fig. 7.115a–d. A 52-year-old man with a smoking history of 25 pack years presented to the emergency department with acute chest pain. a Contrast-enhanced 64-slice CT angiog- raphy of the entire thorax with a gantry rotation time of 330 ms and retrospective ECG gating ruled out acute pulmo- nary embolism as the reason for his chest pain. b Focused reconstruction of the coronary arteries showed diffuse ath- erosclerotic disease. c MIP in a right anterior oblique per- spective of the LAD showed atherosclerotic plaque in the inferior, anteromedial wall of the LAD. The plaque is mildly obstructive and consists of non-calcifi ed tissue adjacent to a calcifi ed nodule. d Analysis of lung-window reconstruc- tions of the entire chest revealed incidental squamous-cell carcinoma of the left upper lobe of the lung
b
d
a c
Fig. 7.116a,b. A 59-year-old woman presenting to the emergency department with acute chest pain. Contrast-enhanced, retro- spectively ECG-gated 64-slice CT study of the entire chest with 0.6-mm collimation was acquired within a total scan time of 19 s. Volume-rendered display of the entire thorax (a) and focused display of the coronary artery tree (b) show normal vascular anatomy, thus ruling out pulmonary embolism, aortic dissection, and coronary artery disease as the underlying cause of chest pain in this patient
b a
undergo the conventional work-up for acute chest pain, but their number should be greatly reduced.
Precious room time in conventional cardiac cath- eter suites, which is currently wasted for performing merely diagnostic angiograms, can be more cost- effectively dedicated to those patients who absolutely require actual intervention. Thus, an overall positive effect on the length of stay and cost to the health-care environment is anticipated.
References
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Flohr T, Prokop M, Becker C, et al (2002). A retrospectively ECG-gated multislice spiral CT scan and reconstruction
Fig. 7.117a–c. A 57-year-old man presenting to the emergency department with acute chest pain. Non-diagnostic ECG, initial cardiac blood markers negative for acute myocardial ischemia. a Contrast-enhanced, retrospectively ECG-gated 64-slice CT study of the entire chest with 0.6-mm collimation was acquired within a total scan time of 21 s. The results ruled out acute pulmonary embolism as a reason for chest pain. b Curved multi-planar reformat shows subtotal thrombosis of the LAD. c Axial MIPs show areas of myocardial hypoattenuation (arrows) in the LAD territory, corresponding to acute myocardial infarction
b
a c
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