A novel, fully 3D, high-resolution T 1 and T 2 relaxation time mapping method is presented. The method is based on steadystate imaging with T 1 and T 2 information derived from either spoiling or fully refocusing the transverse magnetization following each excitation pulse. T 1 is extracted from a pair of spoiled gradient recalled echo (SPGR) images acquired at optimized flip angles. This T 1 information is combined with two refocused steady-state free precession (SSFP) images to determine T 2 . T 1 and T 2 accuracy was evaluated against inversion recovery (IR) and spin-echo (SE) results, respectively. Error within the T 1 and T 2 maps, determined from both phantom and in vivo measurements, is approximately 7% for T 1 between 300 and 2000 ms and 7% for T 2 between 30 and 150 ms. The efficiency of the method, defined as the signal-to-noise ratio (SNR) of the final map per voxel volume per square root scan time, was evaluated against alternative mapping methods. With an efficiency of three times that of multipoint IR and three times that of multiecho SE, our combined approach represents the most efficient of those examined. Acquisition time for a whole brain T 1 map (25 ؋ 25 ؋ 10 cm) is less than 8 min with 1 mm 3 isotropic voxels. An additional 7 min is required for an identically sized T 2 map and postprocessing time is less than 1 min on a 1 GHz PIII PC. A fast and accurate method of determining the longitudinal, T 1 , and transverse, T 2 , relaxation constants on a voxelby-voxel basis has long been a goal of MRI scientists. Rigorous characterization of T 1 and T 2 may allow for greater tissue discrimination, segmentation, and classification, thereby improving disease detection and monitoring, as well as enhancing the images used for image-guided surgical procedures. Absolute determination of T 1 and T 2 is clinically useful in areas such as in-flow perfusion studies (1) and dynamic contrast agent studies (2), as well as in the diagnosis of epilepsy (3) and in determining the severity of Parkinson's disease (4). Therefore, a method that permits simultaneous T 1 and T 2 determination in a rapid manner would be useful in a wide range of imaging applications.In order to be clinically useful for neuroimaging applications, T 1 and T 2 maps should be of high resolution, with a voxel volume less than 1 mm 3 , and have low noise. Imaging time should be less than 30 min for a large volume (25 ϫ 25 ϫ 10 cm) with minimal postprocessing time. Ideally, postprocessing would be performed at the scanner console.Despite the long acquisition times, the principal methods for T 1 and T 2 mapping remain inversion-recovery (IR) and saturation-recovery (SR) for T 1 , and spin echo (SE) and multiple or fast spin echo (mSE, FSE) for T 2 . Although alternative methods (5-9) have been developed to rapidly and accurately determine T 1 or T 2 , the low signal-to-noise ratio (SNR), lengthy reconstruction time, or special hardware requirements associated with these newer methods reduce their appeal.The variable nutation angle method original...

Background and Purpose-The relationship between middle cerebral artery (MCA) flow velocity (CFV) and cerebral blood flow (CBF) is uncertain because of unknown vessel diameter response to physiological stimuli. The purpose of this study was to directly examine the effect of a simulated orthostatic stress (lower body negative pressure [LBNP]) as well as increased or decreased end-tidal carbon dioxide partial pressure (P ET CO 2 ) on MCA diameter and CFV. Methods-Twelve subjects participated in a CO 2 manipulation protocol and/or an LBNP protocol. In the CO 2 manipulation protocol, subjects breathed room air (normocapnia) or 6% inspired CO 2 (hypercapnia), or they hyperventilated to Ϸ25 mm Hg P ET CO 2 (hypocapnia). In the LBNP protocol, subjects experienced 10 minutes each of Ϫ20 and Ϫ40 mm Hg lower body suction. CFV and diameter of the MCA were measured by transcranial Doppler and MRI, respectively, during the experimental protocols. Results-Compared with normocapnia, hypercapnia produced increases in both P ET CO 2 (from 36Ϯ3 to 40Ϯ4 mm Hg, PϽ0.05) and CFV (from 63Ϯ4 to 80Ϯ6 cm/s, PϽ0.001) but did not change MCA diameters (from 2.9Ϯ0.3 to 2.8Ϯ0.3 mm). Hypocapnia produced decreases in both P ET CO 2 (24Ϯ2 mm Hg, PϽ0.005) and CFV (43Ϯ7 cm/s, PϽ0.001) compared with normocapnia, with no change in MCA diameters (from 2.9Ϯ0.3 to 2.9Ϯ0.4 mm). During Ϫ40 mm Hg LBNP, P ET CO 2 was not changed, but CFV (55Ϯ4 cm/s) was reduced from baseline (58Ϯ4 cm/s, PϽ0.05), with no change in MCA diameter. Conclusions-Under the conditions of this study, changes in MCA diameter were not detected. Therefore, we conclude that relative changes in CFV were representative of changes in CBF during the physiological stimuli of moderate LBNP or changes in P ET CO 2 .

The driven-equilibrium single-pulse observation of T 1 (DES-POT1) and T 2 (DESPOT2) are rapid, accurate, and precise methods for voxelwise determination of the longitudinal and transverse relaxation times. A limitation of the methods, however, is the inherent assumption of single-component relaxation. In a variety of biological tissues, in particular human white matter (WM) and gray matter (GM), the relaxation has been shown to be more completely characterized by a summation of two or more relaxation components, or species, each believed to be associated with unique microanatomical domains or water pools. Unfortunately, characterization of these components on a voxelwise, whole-brain basis has traditionally been hindered by impractical acquisition times. In this work we extend the conventional DESPOT1 and DESPOT2 approaches to include multicomponent relaxation analysis. The driven-equilibrium single-pulse observation of T 1 (DESPOT1) and T 2 (DESPOT2) (1,2) methods afford rapid, accurate, and precise evaluation of the longitudinal and transverse relaxation times. As previously described (1,2), unlike the more common spin-echo (SE)-based relaxometry approaches that sample multiple time points along the T 1 recovery or T 2 decay curves, DESPOT1 and DESPOT2 derive T 1 and T 2 information from sets of spoiled gradientrecalled echo (SPGR) and fully-balanced steady-state free precession (bSSFP) data acquired over a range of flip angles, ␣, with constant interpulse spacing, TR. With TR values of less than 10 ms, whole-brain and high-spatialresolution quantitative T 1 and T 2 maps can be acquired in less than 15 min (2,3), a time frame comparable to that of clinical T 1 -or T 2 -weighted acquisitions.Despite the advantages of DESPOT1 and DESPOT2 over alternative relaxometry techniques, both methods are based on the premise that the relaxation of magnetization in each imaging voxel is characterized by a single T 1 and T 2 combination, i.e., that the MR signal arises from a single microanatomical domain or water pool. This proposition, however, overlooks the complex microstructural organization of tissue. Although information related to tissue microstructure has broad clinical utility (for example, in identifying tissue change associated with disease), obtaining such information generally requires invasive or destructive techniques, such as histological analysis. Analysis of transverse relaxation data, however, has shown considerable promise for elucidating tissue microstructure noninvasively by enabling the decomposition of the measured MR signal into multiple components, each believed to originate from distinct tissue subdomains (4 -10). T 2 data obtained from a variety of neural tissues have consistently revealed the presence of at least two relaxation components: a fast-relaxing species with T 2 Ͻ 50 ms, and a slower-relaxing species with T 2 Ͼ 70 ms. Based on histological correlations (11,12), the fast-relaxing species is broadly attributed to water trapped between the lipid bilayers of the myelin sheath, while the s...

Variations in the intrinsic T 1 and T 2 relaxation times have been implicated in numerous neurologic conditions. Unfortunately, the low resolution and long imaging time associated with conventional methods have prevented T 1 and T 2 mapping from becoming part of routine clinical evaluation. In this study, the clinical applicability of the DESPOT1 and DESPOT2 imaging methods for high-resolution, whole-brain, T 1 and T 2 mapping was investigated. In vivo, 1-mm 3 isotropic whole-brain T 1 and T 2 maps of six healthy volunteers were acquired at 1.