The circulation time in the aged

The optimization of scan timing for contrast-enhanced magnetic resonance angiography

OBJECTIVE

To determine the optimal scan timing for contrast-enhanced magnetic resonance angiography and to evaluate a new timing method based on the arteriovenous circulation time.

MATERIALS AND METHODS

Eighty-nine contrast-enhanced magnetic resonance angiographic examinations were performed mainly in the extremities. A 1.5T scanner with a 3-D turbo-FLASH sequence was used, and during each study, two consecutive arterial phases and one venous phase were acquired. Scan delay time was calculated from the time-intensity curve by the traditional (n = 48) and/or the new (n = 41) method. This latter was based on arteriovenous circulation time rather than peak arterial enhancement time, as used in the traditional method. The numbers of first-phase images showing a properly enhanced arterial phase were compared between the two methods.

RESULTS

Mean scan delay time was 5.4 sec longer with the new method than with the traditional. Properly enhanced first-phase images were found in 65% of cases (31/48) using the traditional timing method, and 95% (39/41) using the new method. When cases in which there was mismatch between the target vessel and the time-intensity curve acquisition site are excluded, erroneous acquisition occurred in seven cases with the traditional method, but in none with the new method.

CONCLUSION

The calculation of scan delay time on the basis of arteriovenous circulation time provides better timing for arterial phase acquisition than the traditional method.

MR imaging of hypervascular liver tumors: timing optimization during the arterial phase

To analyze the optimal timing strategy for the detection of hypervascular liver tumors during the arterial phase of magnetic resonance (MR) imaging, a test examination after injection of 2 mL of gadopentetate dimeglumine was performed in 47 patients. The time course of the tumor-to-liver contrast-to-noise ratio (CNR) for all studies together was determined relative to the start of injection, the time of peak aortic enhancement, and the time of peak enhancement in the tumor. All studies were grouped together and the highest CNR was transiently observed at the time of peak tumor enhancement. This CNR was significantly higher than those observed at fixed delays after peak aortic enhancement. However, the CNRs at peak tumor enhancement+/-1.5 seconds did not differ significantly from those obtained after peak aortic enhancement. Finally, the CNRs obtained at fixed delays after the start of injection remained significantly lower. In hypervascular liver tumors, a higher CNR can be obtained during the arterial phase when the MR imaging delay is determined relative to the time of peak enhancement in the tumor or the aorta rather than being fixed after the start of contrast material injection. Timing based on the enhancement profile in the tumor rather than in the aorta should be performed only if rapid MR imaging is available with a time resolution of about 1.5 seconds to image the whole liver.

Aortoiliac and lower extremity arteries: comparison of three-dimensional dynamic contrast-enhanced subtraction MR angiography and conventional angiography

PURPOSE

To determine the clinical feasibility of three-dimensional dynamic contrast agent-enhanced subtraction magnetic resonance (MR) angiography in patients with symptoms of lower extremity ischemia.

MATERIALS AND METHODS

Twenty-three patients suspected of having lower extremity ischemia underwent three-dimensional dynamic contrast-enhanced subtraction MR angiography of the aortoiliac arteries and arteries of the lower extremity. As the reference standard, conventional angiography was also performed. For data analysis, the arterial system was divided into 10 segments. Each segment was classified as normal, mildly stenosed, moderately stenosed, severely stenosed, or occluded.

RESULTS

At conventional angiography, 83 stenosed segments (14 mildly stenosed, 16 moderately stenosed, 14 severely stenosed, and 39 occluded) were identified in a total of 423 segments. For the segments with more than mild stenosis, MR angiography was 97.1% sensitive and 99.2% specific.

CONCLUSION

Three-dimensional dynamic contrast-enhanced subtraction MR angiography has high sensitivity and specificity. This technique is a noninvasive alternative to conventional angiography for screening patients suspected of having lower extremity ischemia.