The Role of Multidetector Computed Tomography Angiography for the Diagnosis of Pulmonary Embolism

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From a radiological point of view, computed tomography pulmonary angiography (CTPA) has effectively become the de-facto first-line imaging test for the evaluation of pulmonary embolism (PE), as patients with a high-quality negative CTPA do not require further examination or treatment for suspected PE. We are likely to see further technical developments in CT technology in the near future. These advances will most likely further improve image quality. Several questions or issues remain, including strategies for further imaging when CT is inconclusive or contraindicated, issues regarding radiation exposure, the prevalence of PE in specific populations, best tests and pathways in specific patient groups, including patients with specific comorbidities such as oncology patients or patients with chronic obstructive pulmonary disease. Also, the question whether all PE patients need anticoagulation, the clinical effect of follow-up imaging, and the accuracy of different clinical prediction rules, remains.

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Ventilation-Perfusion (V/Q) Scintigraphy

V/Q scintigraphy was introduced in 1964 for the evaluation of pulmonary blood flow and has been used as the first-line examination for patients with suspected PE for several decades.2, 7, 8, 9 A high probability scan is sufficient diagnostic evidence of PE to begin anticoagulation therapy, and a normal V/Q scan is considered sufficient evidence to exclude PE. However, the frequency of low or intermediate probability scan results can be as high as 50% to 70%, carrying a 10% to 50% probability of

Computed Tomography

The 1992 report by Remy-Jardin and coworkers was the first to compare helical CT for the evaluation of central PE to selective pulmonary angiography as the reference test, demonstrating 100% sensitivity and 96% specificity in 42 patients.24 At that time, exams were interpreted hard copy, and image collimation was 5-mm on a single detector scanner. As with many first reports, the accuracy estimates may be high because of the selection of more ideal patients for study. Overall, sensitivities for

Computed Tomography

On a per-patient basis, CTPA interobserver agreement for the detection of acute PE is moderate to almost perfect, with kappa values ranging from 0.59 to 0.94.29, 33, 35, 38, 39, 41, 51, 57, 58, 59, 60, 61, 62 Chartrand-Lefebvre and coworkers showed excellent overall interobserver agreement (k = 0.85) and intraobserver agreement (k = 0.87).57 Interobserver agreement was also better in larger vessels, with interobserver agreement at the lobar level (k = 0.70) than at the segmental level (k =

Imaging

In 90% of patients with PE the source of the emboli is the lower-extremity veins (ie, DVT). CTV can be combined with CTPA without requiring any additional intravenous contrast material. Performing CTPA combined with CTV was first described by Loud and coworkers in 1998.70 The same authors subsequently assessed CTV in 71 patients, 19 of whom had DVT revealed on CTV.71 Several studies in which single-detector and MDCT angiography were used have shown that the addition of CTV to the CTPA

Clinical Outcome After a Negative CTPA

When PE is diagnosed by CTPA, specificity is high. Therefore, a positive diagnosis of PE on CT is usually accepted. Several studies have reported that a negative CT pulmonary angiogram for PE is comparable to a negative catheter pulmonary angiogram in terms of patient outcome.26, 30, 33, 43, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100 Thus, in most patients with suspected acute PE and no symptoms, anticoagulation therapy can be safely withheld after negative CTPA.

Computed Tomography

Using an anthropomorphic phantom, Resten and coworkers reported 6.4 ± 1.5 mSv as the mean dose for single-detector CTPA.104 Rademaker and coworkers using a single-detector CT scanner calculated the radiation dose to be approximately 2.2 mSv for the chest.105

MDCT

In most protocols for helical CT of PE, the effective dose is between 3 and 5 mSv, equivalent to 1-2 years of exposure to background radiation. The cancer risk associated with this exposure would be approximately 150 excess cancer deaths per

Preference

In a recent survey of imaging practices for diagnosing acute PE among physicians practicing in the United States that explored factors associated with practice decisions Weiss and coworkers surveyed 855 physicians selected at random from membership lists of 3 professional organizations (general internists, pulmonologists, and emergency medicine specialists) by mail.121 Completed questionnaires were received from 29.8% participants practicing in 44 states. The authors found that 86.7% of

CT Evaluation of Right Ventricular (RV) Dysfunction

The prognosis and optimal therapy in patients with PE are strongly influenced by the presence or absence of hemodynamic compromise. The main cause of death within 30 days from acute PE is RV failure (Fig. 8). Recent evidence indicates that the presence of RV dysfunction identifies a subgroup of normotensive patients with a much more guarded prognosis than patients without RV impairment. Rapid risk stratification is paramount for identifying high-risk patients and helps select the appropriate

ECG-Gated CTA of the Chest

There are a few of reasons that using ECG-gating during CTPA may be useful. An objective assessment of RV function could help stratify patients with RV dysfunction and guide certain therapeutic decisions. Also, the clinical presentation of patients suspected of having acute PE is nonspecific, and it is well established that clinical signs and symptoms of PE and myocardial infarction overlap. Therefore, the possibility of using ECG-gated CT angiography for assessment of coronary artery disease

PIOPED II

The results of the multicenter PIOPED II study, funded by the National Heart, Lung and Blood Institute, were published in June 2006.3 PIOPED II was designed to evaluate the accuracy of MDCT for PE. Patient recruitment began September 2001, with a goal of recruiting 1068 patients. All centers used as a minimum level of technology 4-detector MDCT scanners, and as they acquired scanners with more detector-rows, used those scanners so that the trial results incorporated the best available CT

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