| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Science to Practice |
Department of Radiology, University of British Columbia Hospital, 2211 Wesbrook Mall, Vancouver, BC, Canada V6T 2B5,
bruce.forster@vch.ca
SUMMARY
Functional MR imaging with use of the blood oxygen level–dependent (BOLD) technique allows measurement of skeletal muscle perfusion and may in the future permit noninvasive diagnosis and posttherapeutic follow-up of peripheral arterial occlusive disease (PAOD). In this issue of Radiology, Ledermann et al demonstrate good correlation of muscle BOLD signal response with TcPO2 and laser Doppler flowmetry (LDF) measurements in human volunteers in an induced ischemia–reactive hyperemia model.
THE SETTING
Accurate measurement of skeletal muscle perfusion is an attractive goal in functional imaging, since abnormal microcirculation can be seen in a variety of common diseases, such as diabetes mellitus, compartment syndrome, and PAOD. A reproducible, fast, and noninvasive method of assessing microcirculatory function would not only allow accurate diagnosis but also permit monitoring of endovascular and medical therapies directed at increasing blood flow. Although imaging of peripheral arterial flow has advanced beyond the often inaccurate morphologic assessment to include measurement of pressure gradients and noninvasive flow quantification with use of phase-contrast magnetic resonance (MR) angiography (1), assessment of end-organ (skeletal muscle) function remains an understandably worthwhile target.
|
THE SCIENCE
Various methods have been advanced to assess calf muscle perfusion. Venous occlusion plethysmography is commonly cited as a reference method, but it has not had widespread clinical adoption (1). Perfusion assessment with thallium scintigraphy or positron emission tomography has limited spatial resolution. Other study investigators (3) have used first-pass gadolinium-enhanced techniques to successfully measure tissue perfusion, but the addition of a contrast agent complicates the methodology and is a barrier for repeated analyses. Previous authors (4) have found that muscle BOLD signal intensity changes correlate not only with blood flow at venous occlusion plethysmography but also with muscle perfusion at MR spectroscopy (5).
Ledermann et al (2) performed BOLD imaging with a clinical MR unit in 15 healthy volunteers by using a single-shot echo-planar gradient-echo sequence. They measured the BOLD signal intensity changes in gastrocnemius muscles by using an established ischemia–reactive hyperemia model and compared the time course and amplitudes of the changes with those at LDF and TcPO2 measurement. Good correlation was found between BOLD signal response and findings of the two established techniques, although some differences did exist—particularly in the time course data.
THE PRACTICE
Clinical Use:
Any technique that will have widespread clinical use in the quantitative assessment of PAOD must have high temporal resolution, have sufficient spatial resolution to discern regional perfusion differences in muscle groups, and be noninvasive. BOLD imaging fulfills these criteria, and, as shown by Ledermann et al (2), correlates well with the currently used but semiquantitative and indirect methods LDF and TcPO2 measurement, respectively, in healthy volunteers.
Future Opportunities and Challenges:
Functional imaging can provide valuable information that is ancillary to morphologic evaluation and is likely the key to noninvasive assessment of PAOD. The challenges in such an application of BOLD imaging are several: Because of the formation of collateral vessels, patients with claudication have normal or only slightly reduced blood flow at rest, but during exercise or in induced ischemia models, the small size of the collaterals results in an insufficient increase in blood flow to meet metabolic demand (1). Therefore, functional techniques must be performed under conditions of increased flow demand, such as exercise or induced ischemia. Large veins can be associated with extravascular susceptibility gradients and cause mismapping of BOLD signal intensity and thus must be avoided in the analysis, which itself is cumbersome. Extrapolation of the results in the young healthy subjects examined by Ledermann et al (2) to older patients with PAOD is difficult.
Despite these shortcomings, functional imaging of skeletal muscle with techniques such as BOLD MR imaging represents the appropriate next step in the quantitative assessment of PAOD and may one day also be applied to other organ systems such as the heart, in which atherosclerosis exacts a toll.
FOOTNOTES
See also the article by Ledermann et al in this issue.
References
Related Article
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| RADIOLOGY | RADIOGRAPHICS | RSNA JOURNALS ONLINE |
