Time-resolved MRA useful for stenosis, occlusion evaluation
The addition of time-resolved data to conventionally acquired high-spatial-resolution contrast-enhanced MR angiography (MRA) images results in increased occlusion detection specificity and reader confidence without increasing the time needed for study interpretation, according to researchers at Duke University Medical Center in Durham, N.C.

“One of the primary advantages of time-resolved MIP (maximum intensity projection) imaging is that it enables dynamic visualization of venous blood flow in a manner similar to that in conventional angiography, which facilitates visualization of the enlarged collateral veins that often accompany chronic venous stenoses and occlusions,” wrote the authors of the study, which was published in this month’s issue of Radiology.

According to the researchers, time-resolved MR images are acquired rapidly at short time intervals, which allows for visualization of the temporal dynamics of blood flow. In addition, time-resolved data sets usually require less gadolinium-based contrast material than conventional high-spatial-resolution MRA; a significant consideration for patients with impaired renal function that are at risk for the development of nephrogenic systemic fibrosis.

The research team conducted a retrospective analysis of 27 consecutive patients who underwent gadolinium-enhanced MRA chest imaging to evaluate the central veins.

“At our institution, MRA is the first-line imaging modality used to evaluate superior vena cava syndrome and potential hemodialysis access sites in patients known to have substantial venous disease,” the authors reported

Imaging was conducted on either a 1.5-Tesla (Magnetom Symphony or Avanto; Siemens Medical Solutions) or 3.0-Tesla (Magnetom Tim Trio; Siemens) MR system. A gadolinium-based contrast agent (Magnevist; Berlex) was administered through a peripheral or central venous catheter.

“Three-dimensional imaging data sets were processed into one coronal MIP image for each time point and stored as a cine loop consisting of up to 15 coronal images,” the researchers noted.

Three reading sessions were conducted by six radiologists (two senior-level faculty, two abdominal imaging fellows, and two second-year radiology residents) on a free-standing workstation (Advantage Windows; GE Healthcare): one of the MIP images only; one of the high-spatial-resolution source images; and one of both sets of images. The overall interpretation time per session was 3 minutes for the MIP images, 3.4 minutes for the high-spatial-resolution source images; and 3.3 minutes for both sets of data.

Overall, the scientists reported that the sensitivity for stenoses and occlusion detection was high, without a significant difference between the three image-interpretation sessions.

“However, interpretation of time-resolved MIP images alone resulted in significantly higher sensitivity in the detection of occlusions compared with that in sessions 2 and 3; this was particularly evident in the subset of resident readers (P <.05),” the authors noted.

They also found that the time-resolved MIP images demonstrated superior specificity in the detection of occlusions by the most senior readers. The researchers speculated that the benefit likely resulted from the inference of severe stenoses and occlusions when marked collateral vessels were depicted.

Interestingly, reader confidence was lowest overall for the time-resolved MIP images, higher for the high-spatial-resolution source images, and highest (3.5 on a scale of 1-4) for the combined image sets.

“We believe that the high sensitivity, rapid acquisition time, and need for only a fraction of the overall gadolinium dose should lead to the use of time-resolved MR angiography as an initial evaluation sequence, with the potential to obviate more time-consuming high-spatial-resolution imaging and additional gadolinium-based contrast agent administration,” the authors stated.