Uniform Vascular Enhancement of Lower-Extremity Artery on CT Angiography Using Test-Injection Monitoring at the Central Level of the Scan Range

Rationale and Objectives

To evaluate the efficacy of variable contrast injection durations and scanning delay determined by test injection analysis of computed tomography angiography (CTA) of peripheral arteries.

Materials and Methods

We used a flow phantom that simulates the hemodynamics in a lower extremity artery. We set the flow rate at the pump to 2.0 or 5.0 L/minute. In protocol 1, we adopted a variable contrast injection duration based on the peak enhancement time of the test injection monitoring at the central level of the scan range. In protocol 2, we adopted a fixed contrast injection duration. The scanning delay was determined with a conventional bolus-tracking technique monitoring at the top of the scan range. Mean arterial attenuation and difference between the maximum and minimum attenuation values were calculated. To verify the phantom study results, clinical study, including 16 patients was performed under protocol 1.

Results

The mean attenuation values under protocols 1 and 2 were comparable (563.6 Hounsfield units [HU] and 535.0 HU, respectively) at a pump flow rate of 2.0 L/minute; at 5.0 L/minute, they were 289.4 HU and 328.8 HU. The difference between the maximum and minimum attenuation values was smaller under protocol 1 than protocol 2 (76.8 HU vs. 184.9 HU) at a pump flow of 2.0 L/minute and also smaller under protocol 1 than protocol 2 (79.7 HU vs. 203.8 HU) at 5.0 L/minute. In clinical study, the mean attenuation value was 332.6 ± 51.9 HU, and the difference between the maximum and minimum attenuation values was 55.1 ± 24.4 HU.

Conclusion

The object-specific injection duration based on test injection at the central level of the scan range provides sufficient and constant vascular enhancement at CTA.

Computed tomography angiography (CTA) is an accurate modality to assess the presence and extent of peripheral arterial disease (PAD) . With CTA of peripheral arteries, it is important to evaluate not only arterial stenosis, but also the runoff vessels. Coexisting cardiovascular disorders and blood flow obstruction or aneurysms may delay opacification of peripheral arterial trees . Moreover, the actual flow speed of the injected contrast material through the peripheral arteries is highly variable in patients with PAD . Because high-speed multidetector computed tomography (MDCT) scanning can outpace the flow of the contrast bolus, resulting in inadequate vascular enhancement of peripheral arteries, the contrast injection method should take into account the arterial transit speed.

To achieve satisfactory enhancement of a wide range of aortoiliac and lower extremity arteries, adequate enhancement must be maintained for a certain period depending on the computed tomography (CT) data acquisition time. Time-to-peak arterial enhancement is theoretically equal to the contrast injection duration for a given arterial arrival time when the injection duration is longer than the arterial peak time of the test bolus . Therefore, we applied time-to-peak arterial enhancement of the test injection to the injection duration at CTA to achieve sufficient and constant vascular enhancement. Moreover, we set the monitoring level of the test injection at the central level of the scan range to avoid outpacing the scanning.

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Materials and methods

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Phantom Study

Phantom configuration

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CTA protocols

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Tdelay=Tarrive+Tpeak−1/2Tacquisition, T

delay

=

T

arrive

+

T

peak

−

1

/

2

T

acquisition

,

where T arrive and T peak are the arterial arrival time and the time-to-peak enhancement of the test injection, respectively, and T acquisition is the data acquisition time for CTA. The monitoring level of the test injection was set in the central level of the scan range. We subtracted 1/2 T acquisition from T arrive + T peak for the determination of the delay time. The schematic for protocol 1 is presented in Figure 2 . In protocol 2, we adopted a fixed contrast injection duration (20 seconds) because a fixed injection duration provides constant arterial enhancement regardless of patient weight and injection rate and it is widely used in clinical practice . The scanning delay was determined with a real-time bolus tracking system. The monitoring level was set in the top of the scan range based on the previous clinical studies ( Fig 1 ). The trigger threshold was set at 200 Hounsfield units (HU) for the arterial ROI.

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Data analysis

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Patient Study

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Results

Phantom Study

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Patient Study

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Table 1

Attenuation Values from the Abdominal Aorta to the Lower-extremity Artery in our Patient Study ( n = 16)

Arterial Segment Mean (HU) Range (HU) Abdominal aorta 313.9 (54.8) 249–436 Aortic bifurcation 329.1 (48.9) 281–433 Femoral artery 347.9 (49.5) 291–448 Popliteal artery 354.9 (68.3) 235–467 Posterior tibial artery 318.6 (60.5) 204–441 Average 32.6 (51.9) 274–430

HU, Hounsfield units.

Numbers in parentheses indicate standard deviation.

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Discussion

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References

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