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The effect of hypercapnia on the cerebral metabolic rate of oxygen consumption (CMRO 2 ) remains incompletely understood. This study examined the relationship between susceptibility (blood oxygenation level dependent (BOLD)) and perfusion-weighted (flow-sensitive alternating inversion recovery (FAIR)) MRI techniques both during induction of repeated transient hypercapnia (THC) and after return to normocapnia during whisker barrel functional activation. During induction of THC the FAIR signal became significantly elevated over control after 100 s of hypercapnia (P ‫؍‬ 0.039), with a trend of increasing significance to 5 min (P ‫؍‬ 0.000008). The FAIR signal in the activated cortex during subsequent normocapnia was significantly increased compared to pre-THC control after each successive period of THC. The mean grouped FAIR signal increased by 81% ؎ 63% after one exposure (P ‫؍‬ 0.021), by 163% ؎ 55% after the second exposure (P ‫؍‬ 0.0002), and by 240% ؎ 54% after the third exposure (P ‫؍‬ 0.000002). The mean grouped BOLD signal trended upward, but did not increase significantly during or after exposure 1, 2, or 3. These data demonstrate increased uncoupling of perfusion-weighted from susceptibility imaging techniques, both in nonactivated cortex during hypercapnia, and with activation after multiple exposures to THC. Since blood oxygenation level dependent (BOLD) imaging was first described by Ogawa et al. in 1990 (1,2), it has become an important tool for studies of brain function. However, several questions remain unanswered concerning the BOLD mechanism and the manner in which the functional MRI (fMRI) signal depends on underlying physiological processes. The BOLD signal reflects a complex interplay of the intravascular magnetic susceptibility (which depends on the oxygenation of blood hemoglobin), blood flow, and blood volume. More recently the use of perfusion-weighted imaging (PWI) has increased as a complement to susceptibility imaging. The most commonly used arterial spin labeling methods are related to the flowsensitive alternating inversion recovery (FAIR) sequence, first described by Kim in 1995 (3). A wide array of related research has also been conducted on other determinants of fMRI contrast, such as blood glucose level and the proposed vasodilators nitric oxide and adenosine (4,5,31).One issue of particular interest and importance is the relationship between cerebral blood flow (CBF) and cerebral metabolic rate of oxygen consumption (CMRO 2 ) (6 -9, 30). Various models have been developed to account for this relationship. Buxton and Frank (10) predicted that a very large change in CBF is necessary to supply a small change in CMRO 2 , while Hyder et al. (11,12) predicted a much stronger correlation between CBF and CMRO 2 . One method of investigating this relationship with MRI has been to collect BOLD and PWI data while the subject undergoes graded visual stimulation (13-15). These studies were performed after "calibrating" the functional signal at what was presumed to be baseline CMRO 2 with trans...

Purpose: To retrospectively evaluate the sensitivity, specificity, and positive and negative predictive values of steadystate free-precession (SSFP) survey MRI of the abdomen. Materials and Methods:A total of 375 consecutive outpatients underwent abdominal MRI at 1.5T. Excluding diffuse metastatic disease, 110 patients had at least one other clinically important finding. The SSFP survey included contiguous 5-mm-thick axial, sagittal, and coronal slices (total 90 slices) obtained during a total of 90 seconds of free breathing. Studies were reviewed by two experienced MRI readers independently, randomized, blinded, and at different sittings. The chi-squared test was used to compare SSFP to full MRI for showing clinically important findings. In a subset of 30 patients, confidence intervals (CI) were calculated to compare the accuracy of SSFP and full MRI as predictors of biopsy result.Results: SSFP detected 87.3% of clinically important findings and 93.3% of malignancies reported on the full MRI, with a 1.5% false-positive rate. Significant association was shown between SSFP and full MRI for clinically important findings (P Ͻ 0.0001). Compared to biopsy, accuracy of SSFP was high (85% Ϯ 12.7%), though not as high as full MRI (93.3% Ϯ 8.8%).Conclusion: SSFP provides a rapid survey of the abdomen, with good sensitivity and few false positives.