BOLD MRI measures calf muscle ischemia

Historically, BOLD MR imaging has been used for neurologic purposes. But Swiss researchers have found that oxygenation levels in calf muscle measured by functional MRI correspond to transcutaneous measurement of tissue oxygen tension.

There have been a few reports of blood oxygenation level-dependent (BOLD) MRI for the calf muscles. But the meaning of the BOLD signal in skeletal muscle is not well understood, said Dr. Hans P. Ledermann during a Friday scientific session at the ECR.

"We didn't really know what the muscle BOLD signal meant, whether it was a measure of oxygenation or something else," he said.

Ledermann and colleagues at University Hospital Basel in Switzerland compared the BOLD signal in 18 healthy volunteers (mean age 32 years) with the gold standard of skin oxygen pressure measurement (TcPO2). Subjects were imaged on a 1.5T scanner for a one-minute baseline reference, a six-minute ischemia period, and six minutes after suprasystolic compression of the thigh.

A BOLD-sensitive multiecho echo-planar sequence was used (TE 16, 38, 61, 83 msec/TR 1000 msec/ flip angle 90°) to acquire four 5-mm-thick slices of the calf per second. The researchers used neurological BOLD imaging software to analyze the oxygen levels in the gastrocnemius, peroneal, tibialis anterior, and deep flexor muscles.

The following exponential decrease of BOLD signal occurred during ischemia: gastrocnemius, 7.69%; peroneals, 5.41%; tibialis anterior, 3.49%; and deep flexors, 3.26%. During reactive hyperemia, a steep increase of the BOLD signal occurred with the following peak values: gastrocnemius, 22.68%; peroneals, 13.94%; tibialis anterior, 11.84%; and deep flexors, 19.78%. TcPO2 measurements resulted in identical alterations.

"MR imaging of the calf muscles leads to reproducible signal alterations. The BOLD signal decrease during ischemia correlates with TcPO2 decrease, so it may very well reflect muscular ischemia or hyperischemia," Ledermann said.

As to why different calf muscles have various BOLD signal values, the researchers speculated that it is related to the different vasculature architecture in so-called fast twitch and slow twitch muscles. But that line of research is just beginning, according to Ledermann.

His group also noticed that patients with peripheral arterial occlusive disease have different curves: The ascent is not as steep and the peak lags behind, compared with healthy controls. BOLD imaging may offer a way to see ischemia in these patients, he said.