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Optimization of Combined CT Pulmonary Angiography with Lower Extremity CT Venography

¹ Department of Radiology, New York Presbyterian Hospital—Weill Cornell Medical Center, 525 E. 68th St., New York, NY 10023

 
Presented at the annual meeting of the American Roentgen Ray Society, San Francisco, April-May 1998.

Address correspondence to D. F. Yankelevitz.

² Present address: Department of Radiology, Medical School Hannover, Abt. Diagnostische Radiologie I, 30625 Hannover, Germany.

³ Department of Radiology, Hadassah University Hospital, POB 12000, Jerusalem 91120, Israel.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. We wanted to determine the time delay for maximum enhancement of the deep venous system of the lower extremities after standard CT pulmonary angiography.

SUBJECTS AND METHODS. In 20 patients who had undergone standard CT pulmonary angiography, we measured arterial and venous enhancement at the level of the greater trochanter. These measurements were obtained at 30-sec intervals immediately after completion of CT pulmonary angiography. Ten measurements were obtained in 5 min. Time—density curves were plotted.

RESULTS. We found that the median and average peak venous enhancements were 92 and 95 H, respectively. Time to peak enhancement was variable. Because of the broad shape of the venous time—density curve, near peak enhancement could be achieved in most patients at 2 min after CT pulmonary angiography.

CONCLUSION. CT of the deep venous system of the lower extremities after standard CT pulmonary angiography, performed with appropriate timing considerations, allows near maximal enhancement of the venous system in most patients without altering the optimum CT pulmonary angiography protocol.

Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Pulmonary embolism is the most common acute thoracic emergency in the hospitalized patient. The estimated annual incidence in the United States is approximately 600,000 patients, with one third being missed [1]. Proper treatment is estimated to decrease the mortality by as much as 30%. Pulmonary embolism and deep venous thrombi (DVT) are part of the same disease entity; pulmonary embolism is a consequence of DVT. Patients with symptoms of DVT typically undergo diagnostic studies of the deep venous system. For patients without symptoms of DVT, the diagnosis of thromboembolic disease is more difficult, and numerous algorithms have been suggested [2]. Many radiologists now use helical CT pulmonary angiography [3], but its role relative to ventilation-perfusion scanning is debated.

Between February 1996 and June 1998, we developed a CT protocol to examine both the pulmonary arteries and the deep venous system of the legs by extending the thoracic examination to include the deep veins of the thigh and popliteal fossa. This combined protocol is called CT pulmonary angiography and indirect CT venography. Indirect CT venography does not, like direct CT venography, inject the contrast material directly into the veins in the feet. We present our findings in developing an optimum combined study of CT pulmonary angiography and indirect CT venography.

Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
To determine the time delay that provides maximum enhancement of the deep veins of the legs, we obtained multiple density measurements in 20 patients immediately after CT pulmonary angiography. Institutional review board approval was obtained and patients signed a standard consent form. All scans were performed on a helical scanner (HiSpeed Advantage; General Electric Medical Systems, Milwaukee, WI). Because pulmonary embolism was the primary concern in these patients, CT pulmonary angiography was performed according to our standard protocol, which optimizes pulmonary artery enhancement. We obtained scout images of the chest, pelvis, and lower extremities to the level of the knees. We used a standard exposure of 120 kVp and 240 mA and the helical CT mode with a standard algorithm. Patients were given 140 ml of nonionic iohexol (Omnipaque; Nycomed, Princeton, NJ), 300 mg I/ml at a rate of 3 ml/sec with a 28-sec scan delay. We scanned from the inferior pulmonary veins to the aortic arch, spanning 12-15 cm, with a slice width of 3 mm and a pitch of 1.6 to 1. Images were reconstructed at 1-mm intervals and were viewed on the PACS workstation. Patients were instructed to hyperventilate for five breaths before scanning; if they could not hold their breath for the duration of the scan, they were instructed to breathe out slowly. The remainder of the chest was scanned using the standard chest protocol.

Before CT pulmonary angiography was started, a scanogram and a single axial view of the pelvis at the level of the greater trochanter were taken to establish the unenhanced arterial and venous values of the superficial femoral artery and vein in Hounsfield units. Immediately after CT pulmonary angiography, follow-up scanning every 30 sec was performed at the same level as the unenhanced scan for 5 min. The time of obtaining the first enhanced pelvic image was designated as T₀, and the times of the subsequent scans were designated as T₃₀, T₆₀,..., T₃₀₀. Density measurements for the superficial femoral artery and superficial femoral vein at each time (T₀, T₃₀, T₆₀,..., T₃₀₀) were obtained by placing a circular region of interest centrally within each vessel. The diameter of the region of interest was at least 50% of the diameter of the corresponding vessel. For each patient, arterial and venous time—density curves were plotted for each leg. Peak enhancement and the time to reach this peak were also determined. The time lag between the start of contrast material infusion for the CT pulmonary angiography and the first enhanced pelvic radiograph was approximately 1 min.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Analysis of the arterial and venous enhancement at the level of the greater trochanter attained by the 20 patients who had CT pulmonary angiography revealed that peak arterial enhancement always occurred before peak venous enhancement (Fig. 1). Arterial enhancement rapidly increased within 30 sec for all patients followed by an initial rapid decline that gradually tapered. Venous enhancement was more variable than arterial enhancement. Both the rise to the peak value and the subsequent decline were slower than with arterial enhancement. This difference resulted in a broader time—density curve with overall lower intensities. As arterial intensities declined, they eventually became matched with corresponding venous values. Peak venous enhancement after CT pulmonary angiography ranged from 46 to 161 H (median peak = 92 H, average peak = 95 H).

View larger version (16K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Time-density curve of common femoral artery and vein at level of greater trochanter after completion of CT pulmonary angiography in one patient. Note baseline or unenhanced value. Density measurement at time T0 was obtained immediately after CT pulmonary angiography. Subsequent density measurements were obtained at 30-sec intervals for 5 min. Note difference in shape of two curves. Arterial peak occurs earlier than venous peak and arterial enhancement subsequently decreases more rapidly. Eventually the two curves decrease to same level.

The time to peak venous enhancement was also variable. One patient had already reached his peak venous value at T₀ when scanning of the pelvis was started. Among the remaining patients, five peaked by T₃₀, four by T₆₀, three by T₉₀, four by T₁₂₀, and three by T₁₅₀. To determine the optimal time for measuring venous enhancement to perform the examination in a consistent and easily reproducible manner, we further analyzed the distribution of the densities at the varying time intervals. Because of the relatively gradual change in the time—density curves for the venous system, we found that 85% of patients were within 90% of their peak value at T₁₂₀ (Fig. 2) and that 95% of patients were within 75% of peak enhancement at that time. The median and average enhancement at T₁₂₀ was 86 and 84 H, respectively. Because T₁₂₀ represents a delay of 120 sec after completion of the CT pulmonary angiography portion of the scan (approximately 60 sec), the total delay after start of contrast material infusion for venous CT scanning was approximately 180 sec.

View larger version (42K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —For each patient, time interval (in sec) during which contrast enhancement of common femoral vein remained within 90% of peak value attained (CT scanning every 30 sec from 0 to 360 sec). At time T120, 85% of patients were within 90% of their peak.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Evaluation of DVT using CT has received little attention. In the late 1970s and early 1980s, anecdotal reports described DVT that were detected on routine enhanced abdominal or pelvic CT after upper extremity injection [4, 5]. Recently, direct injection of contrast material into the dorsal veins of the feet, followed by helical CT imaging of the lower extremities, was used to reveal DVT [6, 7]. Less invasive imaging of DVT using a single upper extremity injection for imaging the pulmonary arteries and then continuing more caudad to image the pelvis and lower extremities has recently been described [8, 9, 10]. This technique avoids direct injection into the leg veins and allows evaluation of both pulmonary embolism and DVT (Fig. 3). DVT can be detected on routine contrast-enhanced pelvic CT although limited information on the thrombus density is available [3, 4]. One recent study reported that acute thrombi, clinically judged to be less than 8 days old, have an average attenuation value of 66 H whereas those older than 8 days have a lower value of 55 H [6].

View larger version (70K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3. —58-year-old woman with deep venous thrombi (DVT) in common femoral vein after CT pulmonary angiography. CT scan shows that degree of venous enhancement is clearly sufficient to visualize thrombus (arrow). DVT was confirmed by subsequent lower extremity venous Doppler sonography (not shown).

The greater the venous enhancement, the greater the likelihood of detection of DVT and acute DVT. Analysis of the time—density curves of the superficial femoral vein at the level of the greater trochanter after CT pulmonary angiography showed that, on average, the peak enhancement was 95 H. We also found that, unlike arterial enhancement that rapidly reaches its peak value, venous enhancement increases slowly and has a gradual decline. This variation allows most patients to be scanned close to their peak venous enhancement. The time the peak venous enhancement was reached was highly variable among the patients. A time delay of 120 sec after completion of CT pulmonary angiography still allowed 85% of the patients to be within 90% of their peak venous enhancement and 95% of the patients to be within 75% of their peak value.

Analysis of the shape of the time—density curve shows several advantages for CT pulmonary angiography and indirect CT venography. Most importantly, the optimum time for the start of venous scanning occurs after the completion of the optimum CT pulmonary angiography. This starting time allows optimum scanning of both the pulmonary arteries and the deep venous system of the lower extremities. Furthermore, the broad venous peak permits optimization of the venous scanning parameters so that the start of venous scanning can be chosen near the maximal enhancement value for most patients. Because venous enhancement decreases relatively slowly, more time is available to perform indirect CT venography. This greater flexibility allows the spatial resolution to be optimized by using either thinner CT sections or a lower pitch, both of which reduce volume averaging. This reduction is an important benefit of performing the indirect CT venography as compared with the direct CT venography when contrast material is injected directly into the foot veins. The time to peak venous enhancement is short in direct CT venography, and the lower extremities must be scanned quickly using thick sections with a high pitch.

A further advantage of combined CT pulmonary angiography and indirect CT venography is elimination of the need to force contrast material from the superficial to the deep venous system using leg elevation, tourniquets, or ace bandages. Contrast material injected into an arm vein for CT pulmonary angiography enters the arterial system before reaching the leg veins and thus drains directly into the deep venous system of the legs. Thus, occlusion of the superficial veins has little effect on venous enhancement. On the other hand, direct injection into the feet causes the contrast material to enter the superficial veins directly so that they must be compressed for the deep venous system to become opacified.

The only significant disadvantage of indirect CT venography compared with direct CT venography is that its peak venous enhancement is lower. Further evaluation will be required to determine the sensitivity, specificity, and value of this technique when used with CT pulmonary angiography. On the basis of our results, we continue to perform CT pulmonary angiography according to our routine protocol (defined under Subjects and Methods). Three minutes after the start of contrast material infusion, we begin scanning from the iliac crest to the knee. This time delay after CT pulmonary angiography should remain consistent for different CT pulmonary angiography protocols because the time to peak venous enhancement depends primarily on the patient's circulation.

References

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