Near-infrared (NIR) spectroscopy is a noninvasive technique that uses the differential absorption properties of hemoglobin to evaluate skeletal muscle oxygenation. Oxygenated and deoxygenated hemoglobin absorb light equally at 800 nm, whereas at 760 nm absorption is primarily from deoxygenated hemoglobin. Therefore, monitoring these two wavelengths provides an index of deoxygenation. To investigate whether venous oxygen saturation and absorption between 760 and 800 nm (760-800 nm absorption) are correlated, both were measured during forearm exercise. Significant correlations were observed in all subjects (r = 0.92 +/- 0.07; P < 0.05). The contribution of skin flow to the changes in 760-800 nm absorption was investigated by simultaneous measurement of skin flow by laser flow Doppler and NIR recordings during hot water immersion. Changes in skin flow but not 760-800 nm absorption were noted. Intra-arterial infusions of nitroprusside and norepinephrine were performed to study the effect of alteration of muscle perfusion on 760-800 nm absorption. Limb flow was measured with venous plethysmography. Percent oxygenation increased with nitroprusside and decreased with norepinephrine. Finally, the contribution of myoglobin to the 760-800 nm absorption was assessed by using 1H-magnetic resonance spectroscopy. At peak exercise, percent NIR deoxygenation during exercise was 80 +/- 7%, but only one subject exhibited a small deoxygenated myoglobin signal. In conclusion, 760-800 nm absorption is 1) closely correlated with venous oxygen saturation, 2) minimally affected by skin blood flow, 3) altered by changes in limb perfusion, and 4) primarily derived from deoxygenated hemoglobin and not myoglobin.

No abstract

The objective of this study was to investigate the feasibility of whole-body imaging at 7T. To achieve this objective, new technology and methods were developed. Radio frequency (RF) field distribution and specific absorption rate (SAR) were first explored through numerical modeling. A body coil was then designed and built. Multichannel transmit and receive coils were also developed and implemented. With this new technology in hand, an imaging survey of the "landscape" of the human body at 7T was conducted. Cardiac imaging at 7T appeared to be possible. The potential for breast imaging and spectroscopy was demonstrated. Preliminary results of the first human body imaging at 7T suggest both promise and directions for further development. Magn Reson Med 61:244 -248, 2009.

The use of body coils is favored for homogeneous excitation, and such coils are often paired with surface coils or arrays for sensitive reception in many MRI applications. While the body coil's physical size and resultant electrical length make this circuit difficult to design for any field strength, recent efforts to build efficient body coils for applications at 3T and above have been especially challenging. To meet this challenge, we developed an efficient new transverse electromagnetic (TEM) body coil and demonstrated its use in human studies at field strengths up to 4T. Head, body, and breast images were acquired within peak power constraints of <8 kW. Bench studies indicate that these body coils are feasible to 8T. RF shimming was used to remove a high-field-related cardiac imaging artifact in these preliminary studies. Whole-body imaging at field strengths up to 4T was investigated by MR system manufacturers in the late 1980s (1,2). However, it was thought that the RF power requirements were too high, and RF penetration was too low for successful body imaging above 100 MHz (3). As with most "first" results, the earliest images acquired at higher fields were not the "best" results. Skepticism about high-field body imaging followed, and persisted for nearly a decade (4). Not until recently have technology, support, and interest coalesced to produce encouraging results at fields higher than 1.5 T. Preliminary body imaging results acquired with 55-60-cm inner diameter (i.d.) birdcage transmit coils together with phased-array receivers indicate that 3T whole-body systems may have a clinical role (5). However, these developments are still in their infancy, and there are remaining problems to be solved and performance parameters to be improved. Commercial 128-MHz body coils are usually paired with 35-kW RF power amplifiers to compensate for increased RF losses to the patient and the coil. This is a significant peak power increase compared to 1.5 T MRI, where 15-20-kW amplifiers are more typical. Accordingly, the scope and scale of image protocols operating within FDA specific absorption rate (SAR) guidelines for 3T are highly constrained compared to those for 1.5T.By this trend, body imaging at 4T would be more limited still and would require amplifiers approaching a 50-kW peak. Thus, if the human body provided the only significant loss mechanism for imaging at 3T with coil designs requiring 35 kW of peak RF power, body coils and body imaging above 3T would be improbable. However, if it can be shown that RF coil losses contribute a significant fraction of the total loss, then the use of more efficient coil circuits might lessen the total RF losses and make human imaging feasible at 4T and higher.