T1 and T2 measurements on a 1.5-T commercial MR imager.

661

815

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...

Section: Resultsmentioning

confidence: 66%

Rapid combined T₁ and T₂ mapping using gradient recalled acquisition in the steady state

Deoni

Rutt

Peters

2003

661

815

“…T 1 of CSF ∼4 s), the longitudinal magnetization is effectively zero at the end of the FSE train, a condition required for the SR method. The FSE train had an echo time of ∼500 ms, more than five times longer than the transverse relaxation time T 2 of brain tissue (∼90 ms (20)(21)(22)) but much shorter than the T 2 of CSF (∼2600 ms (22)); thus the MR signals recorded arise almost entirely from the CSF. This T 2 -weighting effect automatically segments the tissue out of the image and reduces the error in R 1 measurement of CSF when a pixel is shared by CSF and tissue (3).…”

Section: Mrimentioning

confidence: 99%

Optimizing saturation–recovery measurements of the longitudinal relaxation rate under time constraints

Hsu

Glover

Zaharchuk

2009

The saturation-recovery method using two and three recovery times is studied for conditions in which the sum of recovery times is 1.5T 1 to 3T 1 , where T 1 is the longitudinal relaxation time. These conditions can reduce scan time considerably for long T 1 species and make longitudinal relaxation rate R 1 (R 1 = 1/T 1 ) mapping for body fluids clinically feasible. Monte Carlo computer simulation is carried out to determine the ideal set of recovery times under various constraints of the sum of recovery times. The ideal set is found to be approximately invariant to the signal-to-noise ratio. For the three-point method, two of the recovery times should be set the same or approximately the same and should be shorter than the third one. Only marginal improvements in accuracy and precision can be achieved by the three-point method over the two-point method under a common constraint of the sum of recovery times. Three-dimensional, high resolution, whole-brain saturation-recovery scans on volunteers with a fast-spin-echo technique ( As demonstrated in a series of recent studies (1-4), mapping the magnetization longitudinal relaxation rate R 1 of body fluids by MRI holds great potential to be a method of determining the oxygenation (oximetry) of the fluids in vivo. The fluids of interest, e.g., urine, vitreous humor, cerebrospinal fluid (CSF), have long relaxation time constants T 1 (T 1 = 1/R 1 ) in comparison with tissues; thus when the imaging time parameters that are optimal for relaxation measurement of tissue (T 1 ∼ 1 s) are scaled for the body fluids (T 1 ∼ 4 s), the total scan time can be too long to be clinically feasible. All existing clinical methods of oximetry are highly invasive, requiring either microelectrode or optode insertion, and can obtain only point sample measurements. MRI oximetry is hence desirable for its noninvasiveness and volume coverage. However, the long scan time is a major obstacle in translating MRI oximetry of body fluids to the clinic, especially when the time allocated to R 1 mapping is only a portion of a radiological examination. Earlier efforts (5-9) to optimize MRI parameters were focused on the stability of measurement precision against noise influence or on optimizing for efficiency in terms of precision per unit of time. When the total scan time is further subject to a constraint, optimization of imaging time parameters remains to be studied. The saturation-recovery (SR) method (10) is a common technique for R 1 measurement, in which the longitudinal magnetization is set to zero and then measured after a delay time τ . The delay time τ is referred to as the recovery time. The value of R 1 is determined by curve-fitting separate measurements with different recovery times. SR does not need to wait for full magnetization recovery before the next repetition, a time-saving feature compared with the inversion-recovery method (11), and it has the signal-tonoise ratio (SNR) advantage of using large radio-frequency pulse flip angle (90• ) for signal acquisition in comparison with puls...

“…Retrospective first-order shim correction was performed using a B 0 -field map calculated for each slice from the first two frames with different echotimes (⌬TE ϭ 2 ms) resulting in an average root mean square (RMS) of 5.9 and 8.1 Hz over the head area at 1.5 T and 3.0 T, respectively. In order to keep BOLD-sensitivity and susceptibility effects in GRE-imaging at the higher field unchanged, we scaled TE and the flip angle with respect to T 1 and T * 2 properties at 1.5 T and 3.0 T (15)(16)(17).…”

Section: Imaging and Scaling Of Sequence Parametersmentioning

confidence: 99%

“…Thus, specific care has to be taken during the design of imaging experiments, as an improper choice of sequence parameter can easily diminish the promised gain in SNR. Notably, the T 1 relaxation time in brain tissue increases with field strength (15,17,18,20) and thus counteracts the gain from in SNR and CNR in GRE sequences with TR Ӷ T 1 at higher fields (see Theory). Beyond that, we employed very similar sampling times at both fields (T s ϭ 40.1 ms and T s ϭ 39.8 ms).…”

Section: Snrmentioning

confidence: 99%

Neuroimaging at 1.5 T and 3.0 T: Comparison of oxygenation‐sensitive magnetic resonance imaging

Krüger

Kastrup

Glover

2001

312

222

Noise properties, the signal-to-noise ratio (SNR), contrast-tonoise ratio (CNR), and signal responses were compared during functional activation of the human brain at 1.5 and 3.0 T. At the higher field spiral gradient-echo (GRE) brain images revealed an average gain in SNR of 1.7 in fully relaxed and 2.2 in images with a repetition time (TR) of 1.5 sec. The tempered gain at longer TRs reflects the fact that the physiological noise depends on the signal strength and becomes a larger fraction of the total noise at 3.0 T. Activation of the primary motor and visual cortex resulted in a 36% and 44% increase of "activated pixels" at 3.0 T, which reflects a greater sensitivity for the detection of activated gray matter at the higher field. The gain in the CNR exhibited a dependency on the underlying tissue, i.e., an increase of 1.8؋ in regions of particular high activation-induced signal changes (presumably venous vessels) and of 2.2؋ in the average activated areas. These results demonstrate that 3.0 T provides a clear advantage over MRI modalities are often limited by the signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR). Both SNR and CNR have been shown to increase with magnetic field strength B 0 (1,2). Consequently, the "optimal field strength" and the field dependency in blood oxygenation level dependent (BOLD) MRI have been the subject of various investigations (1,3-6). For many MRI applications a magnetic field strength of 1.5 Tesla (T) seems to represent a good compromise. Functional MRI (fMRI), however, is particularly dependent on good SNR and CNR properties, since typically observed BOLD signal changes at 1.5 T are on the order of a few percent and often exceed the intrinsic noise only slightly. Several biophysical models of activation-induced changes of the oxygenation-sensitive MRI signals have proposed that the changes in the relaxation rate ⌬R* 2 and subsequently the BOLD effect are proportional to B 0 for large vessels and proportional to B 0 2 for small vessels and capillaries (7,8). Thus, higher fields may provide an important improvement in fMRI. Indeed, recent investigations have demonstrated a superlinear increase in the BOLD CNR with the field strength (1,4,5), suggesting that high field fMRI methods may be able to resolve oxygenation changes in small vessels and capillaries, which are spatially localized near the origin of the neuronal activity.In the present study, various BOLD-relevant properties were compared at 1.5 T and 3.0 T. In order to establish identical BOLD-sensitivities, we investigated the T* 2 relaxation times for gray matter at each field strength and scaled the corresponding echo time (TE), and the excitation angle at 3.0 T. We compared intrinsic noise contributions and the SNR in gradient-echo (GRE) images and examined activation-induced BOLD responses during visual and motor activation at both fields in terms of spatial extent, the mean z-score, and the CNR of "activated voxels." T* 2 -maps from various brain sections were calculated to investigate spatial aspect...