Pulmonary Embolism Case Study
Clinical History
A patient was admitted for elective abdominal surgery and developed respiratory distress on the 2nd post operative day. Heart and respiratory rates were both high, but without obvious cyanosis. Pulmonary embolism was suspected.
The patient was referred for a SPECT•CT 99mTc MAA lung perfusion study combined with a pulmonary CT angiogram for a comprehensive evaluation of the extent and severity of a pulmonary embolus.
Imaging Findings
Examination Protocols: The lung perfusion study was performed after an intravenous injection of 5 mCi of 99mTc MAA. Planar images were acquired, followed by a SPECT•CT scan. The CT angiography was performed following intravenous injection of 80 cc of iodinated contrast at a flow rate of 3 ml/sec using a pressure injector. Breathold CT acquisition started 20 sec following the start of the contrast injection.*
The SPECT study showed an absence of perfusion in the whole left lung and in the middle and lower lobes of the right lung. The upper lobe of the right lung showed normal perfusion but with small peripheral perfusion defects. The study suggested large pulmonary emboli in the left main pulmonary artery and in the right lower pulmonary artery with small peripheral emboli in the right upper lobe.
The CT pulmonary angiography showed a large pulmonary embolus in the left main pulmonary artery extending into the secondary as well as tertiary branches of the left pulmonary artery. The inferior branch of the right pulmonary artery also showed a large pulmonary embolus with an extension into the tertiary branches. The upper branch of the right pulmonary artery showed normal blood flow which explained the relatively normal perfusion in the right upper lobe of the lung. Peripheral emboli in the right upper lobar branches could not be demonstrated in the CT angiogram although small peripheral perfusion defects in the right upper lobe of the lung were demonstrated on the lung perfusion scintigraphy SPECT study.
The SPECT•CT study with fusion of SPECT with CT angiography performed as an integrated study demonstrated severe pulmonary embolism involving both pulmonary arteries with total absence of perfusion in the left lung and middle and lower lobe of the right lung.
The upper lobe of the right lung showed normal perfusion with a few peripheral small defects probably related to small peripheral emboli.
Treatment
The patient was diagnosed with a bilateral severe pulmonary embolism with extension of the embolus into the secondary and tertiary branches. Because there was a relative sparing of the right upper pulmonary artery with only the right upper lobes perfused, the patient was referred immediately for surgery. The patient underwent an open thoracotomy with an embolectomy and after recovery, there was a gradual improvement in the arterial blood gas profile and oxygenation. The patient was discharged from the hospital with a nearly complete recovery and with follow-up instructions.
Discussion
The role of Ventilation/Perfusion (V/Q) SPECT and multislice CT angiography in the diagnosis of pulmonary embolism has been widely studied. A study performed on 94 patients comparing multislice CT angiography, planar V/Q and pulmonary angiography showed slightly higher diagnostic accuracy of thin slice multislice CT contrast angiography (1). CT angiography had a sensitivity and specificity of 96% and 98% respectively. Planar V/Q SPECT showed 86% sensitivity and 88% specificity. In view of the higher image quality and resolution of V/Q SPECT, it was deemed necessary to compare multislice CT angiography with SPECT in order to truly understand the relative accuracies. One study analyzing 84 patients with suspected pulmonary embolism concluded that V/Q SPECT had higher sensitivity, while CT angiography showed higher specificity. The increased sensitivity of SPECT was predominantly related to small subsegmental pulmonary emboli which were often missed on CT angiography (2). V/Q SPECT was much more sensitive for subsegmental lesions as compared to planar SPECT.
The capability of combining V/Q SPECT with multislice CT angiography, as possible on the Symbia SPECT•CT system, suddenly opens a new approach to the non-invasive diagnosis of pulmonary embolism. To the author’s knowledge, this case is the first in which V/Q SPECT and CT contrast pulmonary angiography were performed in the same setting on an integrated hybrid imaging system, combining SPECT with multislice spiral CT. The combined approach was effective not only in the diagnosis of the presence, the extent and the severity of the pulmonary embolism, but also in the determination of the extent of perfused lung tissue. The integrated multimodality diagnosis was instrumental in the decision to immediately operate on the patient without any additional investigation, including catheter pulmonary angiography. The favorable outcome for this patient reinforces the value of the diagnostic approach of integrating SPECT and diagnostic multislice CT, which led to quicker, more comprehensive and more accurate diagnosis, as well as faster therapeutic decision making.
Data courtesy of Dr. Shahid Mahmood of Gleneagles Hospital, Singapore
References:
1. Diagnosis of acute Pulmonary embolism in Outpatients: Comparison of Thin-Collimation Multidetector row spiral CT and Planar Ventilation Perfusion scintigraphy - Choche et al. - Radiology 2003;229:757-765
2. Tomographic imaging in the diagnosis of Pulmonary Embolism: A comparison between V/Q Lung scintigraphy SPECT and Multislice Spiral CT - Reinartz et al. - JNM 2004:45:1501-1508
*Any of the protocols presented herein are for informational purposes and are not meant to substitute for any clinicians' judgment in how best to use any medical devices. It is the clinician that makes all diagnostic determinations based upon education, learning and experience.