Investigating the effect of exchange and multicomponent T(1) relaxation on the short repetition time spoiled steady-state signal and the DESPOT1 T(1) quantification method

PURPOSE: To examine the spoiled steady-state (spoiled gradient-recalled echo sequence [SPGR]) signal arising from two-compartment systems and the role of experimental parameters, in particular TR for resolving signal from each compartment. MATERIALS AND METHODS: Using Bloch-McConnell simulations, we examined the SPGR signal from two-component systems in which T(1) is much greater than the mean residence time (tau(m)) of proton spins in each component. Specifically, we examined the role of TR on the ability to resolve each components signal, as well as the influence of experimental parameters on derived DESPOT1 T(1) values. RESULTS: Results revealed that when TR < or = 0.01 tau(m), the measured SPGR signal may be modeled as a summation of signal from each species using a no-exchange approximation. Additionally, under this short TR condition, the driven equilibrium single pulse observation of T(1) (DESPOT1) mapping approach provides T(1) values preferentially biased toward the short or long T(1) species, depending on the choice of flip angles. CONCLUSION: The ability to model the SPGR signal using a no-exchange approximation may permit the quantification multicomponent T(1) relaxation in vivo. Additionally, the ability to preferentially weight the DESPOT1 T(1) value toward the short or long T(1) may provide a useful window into these components.     

Change in the proton T1 of fat and water in mixture

This work describes observed changes in the proton T₁ relaxation time of both water and lipid when they are in relatively homogeneous mixtures. Results obtained from vegetable oil–water emulsions, pork kidney and lard mixtures, and excised samples of white and brown adipose tissues are presented to demonstrate this change in T₁ as a function of mixture fat fraction. As an initial proof of concept, a simpler acetone‐water experiment was performed to take advantage of complete miscibility between acetone and water and both components' single chemical shift peaks. Single‐voxel MR spectroscopy was used to measure the T₁ of predominant methylene spins in fat and the T₁ of water spins in each setup. In the vegetable oil–water emulsions, the T₁ of fat varied by as much as 3‐fold when water was the dominant mixture component. The T₁ of pure lard increased by 170 msec (+37%) when it was blended with lean kidney tissue in a 16% fatty mixture. The fat T₁ of lipid‐rich white adipose tissue was 312 msec. In contrast, the fat T₁ of leaner brown adipose tissue (fat fraction 53%) was 460 msec. A change in the water T₁ from that of pure water was also observed in the experiments. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

7 Tesla MR imaging of the human eye in vivo

Purpose:

To develop a protocol which optimizes contrast, resolution and scan time for three‐dimensional (3D) imaging of the human eye in vivo using a 7 Tesla (T) scanner and custom radio frequency (RF) coil.

Materials and Methods:

Initial testing was conducted to reduce motion and susceptibility artifacts. Three‐dimensional FFE and IR‐TFE images were obtained with variable flip angles and TI times. T₁ measurements were made and numerical simulations were performed to determine the ideal contrast of certain ocular structures. Studies were performed to optimize resolution and signal‐to‐noise ratio (SNR) with scan times from 20 s to 5 min.

Results:

Motion and susceptibility artifacts were reduced through careful subject preparation. T₁ values of the ocular structures are in line with previous work at 1.5T. A voxel size of 0.15 × 0.25 × 1.0 mm³ was obtained with a scan time of approximately 35 s for both 3D FFE and IR‐TFE sequences. Multiple images were registered in 3D to produce final SNRs over 40.

Conclusion:

Optimization of pulse sequences and avoidance of susceptibility and motion artifacts led to high quality images with spatial resolution and SNR exceeding prior work. Ocular imaging at 7T with a dedicated coil improves the ability to make measurements of the fine structures of the eye. J. Magn. Reson. Imaging 2009;30:924–932. © 2009 Wiley‐Liss, Inc.     

Suitability of T(1Gd) as the dGEMRIC index at 1.5T and 3.0T.

Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) is based on the theory that Gd-DTPA(2-) will distribute in inverse relation to cartilage glycosaminoglycan (GAG). T(1Gd) (T(1) after penetration of a 0.2 mmol/kg dose of Gd-DTPA(2-)) has been used as the dGEMRIC index, although (1/T(1Gd)-1/T(1o)) should be more representative of Gd-DTPA(2-) concentration (where T(1o) = T(1) before contrast). T(1o) and T(1Gd) were measured in 20 volunteers at both 1.5T and 3T and the correlation between the metrics of T(1Gd) and (1/T(1Gd)-1/T(1o)) was calculated. There was a high correlation coefficient between the two metrics at both field strengths, with R = 0.94, 0.93, and 0.90 for central medial femur, posterior medial femur, and medial tibia, respectively, at 1.5T and 0.87, 0.94, 0.96 at 3T. In all cases P < 0.0001. Therefore, these data suggest that, for native cartilage, the current practice of measuring T(1Gd) (but not also T(1o)) is adequate at both 1.5T and 3T.     

Fast high-resolution T1 mapping of the human brain.

A sequence for the acquisition of high-resolution T1 maps, based on magnetization-prepared multislice fast low-angle shot (FLASH) imaging, is presented. In contrast to similar methods, no saturation pulses are used, resulting in an increased dynamic range of the relaxation process. Furthermore, it is possible to acquire data during all relaxation delays because only slice-selective radiofrequency (RF) pulses are used for inversion and excitation. This allows for a reduction of the total acquisition time, or scanning with a reduced bandwidth, which improves the signal-to-noise ratio (SNR). The method generates quantitative T1 maps with an in-plane resolution of 1 mm, slice thickness of 4 mm, and whole-brain coverage in a clinically acceptable imaging time of about 19 s per slice. It is shown that the use of off-center RF pulses does not result in imperfect inversion or magnetization transfer (MT) effects. In addition, an improved fitting algorithm based on smoothed flip angle maps is presented and tested successfully.     

T1, T2 and proton density measurements in the grading of cerebral gliomas

The possibility that cerebral tumours may be graded by measuring T1 or T2 with magnetic resonance (MR) imaging was studied. A consecutive series of patients with subsequently verified gliomas was enrolled, and studied with MR. Patients who had prior surgical, chemotherapeutic or steroid treatment were excluded. Single slice multiple saturation recovery and multiple spin echo techniques were used to measure T1, T2 and proton density in the tumour. In 33 patients with cerebral gliomas there were 5 grade I, 12 grade II, 7 grade III and 9 grade IV. T1 and T2 values tended to be smaller in grade I gliomas than in grades II, III and IV gliomas. Relaxation parameters overlapped considerably in tumours with different grades. Proton density values did not show much change between different grades of gliomas. Relaxation parameters cannot be used to determine tumour grade reliably. Correspondence to: S. Newman     

以低场磁共振为首诊急性基底节区脑出血13例影像分析

目的探讨低场强MRI对于基底节区脑出血T1、T2加权像(T1WI、T2WI)信号的影像特征,减少误诊。方法对13例以低场强磁共振为首诊的脑出血患者作常规横断位SE T1WI、FSE T2WI和T1FLAIR、T2 FLAIR序列扫描。结果与结论低场MRI对急性基底节区脑出血的诊断有特异性,充分认识,可以避免误诊。     

T1 mapping of the gadolinium‐enhanced myocardium: Adjustment for factors affecting interpatient comparison

Quantitative T₁ mapping of delayed gadolinium‐enhanced cardiac magnetic resonance imaging has shown promise in identifying diffuse myocardial fibrosis. Despite careful control of magnetic resonance imaging parameters, comparison of T₁ times between different patients may be problematic because of patient specific factors such as gadolinium dose, differing glomerular filtration rates, and patient specific delay times. In this work, a model driven approach to account for variations between patients to allow for comparison of T₁ data is provided. Kinetic model parameter values were derived from healthy volunteer time‐contrast curves. Correction values for the factors described above were used to normalize T₁ values to a matched state. Examples of pre‐ and postcorrected values for a pool of normal subjects and in a patient cohort of type 1 diabetic patients shows tighter clustering and improved discrimination of disease state. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

FLAIR T1序列在颅脑MRI中的应用评估

目的　探讨FLAIRT1序列的临床应用价值。方法　颅脑正常 2 0例 ,病变者 10例 ,分别进行FLAIRT1和SE序列T1加权扫描对比分析。结果　FLAIRT1序列上灰质与白质对比好 ,基底节核团显示清楚 ;FLAIRT1序列与SET1加权像对病灶范围的显示相似 ,但FLAIRT1显示病变与水肿及周围正常脑组织的对比度高于SE序列T1加权像。结论　随着FLAIRT1序列的改进 ,它有望成为T1加权像一个有价值的补充     

Role of magnetic resonance imaging in pre-operative assessment of ano-rectal fistula

Aim of the work

To evaluate the role of magnetic resonance imaging (MRI) in preoperative assessment of ano-rectal fistula and tracing its full extent and relationship.

Materials and methods

Twenty-four patients with ano-rectal fistula were enrolled in this study. They were examined with different MRI sequences for evaluation of the fistulas and their extent. Fistulas were classified according to St. James’s University Hospital MRI based classification system (which correlates the Parks surgical classification to anatomic MRI findings) into 5 grades. Then, interrelation between surgical and MRI findings was statistically analyzed with evaluation of the accuracy of each MRI sequence used.

Results

Grade 1 was the most frequent (37.5%) type of ano-rectal fistula. The most common location of the internal opening of the fistula was at 6 o’clock position. Combination of oblique coronal and axial planes of contrast-enhanced fat suppressed T1-weighed fast spin-echo (CE FS T1WFSE) sequence images showed the highest accuracy (99.4%) in diagnosis of ano-rectal fistula.

Conclusion

MRI is a useful imaging tool in the preoperative assessment of ano-rectal fistula. A significant accordance between surgical and MRI findings was achieved by using combination of coronal and axial planes of CE FS T1WFSE sequence images.     

Optimization and validation of a fully-integrated pulse sequence for modified look-locker inversion-recovery (MOLLI) T1 mapping of the heart

PURPOSE: To optimize and validate a fully-integrated version of modified Look-Locker inversion-recovery (MOLLI) for clinical single-breathhold cardiac T1 mapping. MATERIALS AND METHODS: A MOLLI variant allowing direct access to all pulse sequence parameters was implemented on a 1.5T MR system. Varying four critical sequence parameters, MOLLI was performed in eight gadolinium-doped agarose gel phantoms at different simulated heart rates. T1 values were derived for each variant and compared to nominal T1 values. Based on the results, MOLLI was performed in midcavity short-axis views of 20 healthy volunteers pre- and post-Gd-DTPA. RESULTS: In phantoms, a readout flip angle of 35 degrees , minimum TI of 100 msec, TI increment of 80 msec, and use of three pausing heart cycles allowed for most accurate and least heart rate-dependent T1 measurements. Using this pulse sequence scheme in humans, T1 relaxation times in normal myocardium were comparable to data from previous studies, and showed narrow ranges both pre- and postcontrast without heart rate dependency. CONCLUSION: We present an optimized implementation of MOLLI for fast T1 mapping with high spatial resolution, which can be integrated into routine imaging protocols. T1 accuracy is superior to the original set of pulse sequence parameters and heart rate dependency is avoided.     

Quantitative regional oxygen transfer imaging of the human lung

PURPOSE: To demonstrate that the use of nonquantitative methods in oxygen-enhanced (OE) lung imaging can be problematic and to present a new approach for quantitative OE lung imaging, which fulfills the requirements for easy application in clinical practice. MATERIALS AND METHODS: A total of 10 healthy volunteers and three non-small-cell lung cancer (NSCLC) patients were examined using a 1.5T scanner. OE imaging was performed using a snapshot fast low-angle shot (FLASH) T(1)-mapping technique (TE = 1.4 msec, TR = 3.5 msec) as well as a series of T(1)-weighted inversion recovery (IR) half- Fourier acquisition single-shot turbo spin-echo (HASTE) (TE(effective) = 43 msec, TE(inter) = 4.2 msec, and inversion time [TI] = 1200 msec) images. Semiquantitative relative signal enhancement ratios (RER) of T(1)-weighted images before and after inhalation of oxygen-enriched gas were compared to the quantitative change in T(1). A hybrid method is proposed that combines the advantages of T(1)-weighted imaging with the quantification provided by T(1)-mapping. To this end, the IR-HASTE images were transformed into quantitative parameter maps. To prevent mismatching and incorrect parameter maps, retrospective image selection was performed using a postprocessing navigator technique. RESULTS: The RER was dependent on the intrinsic values of T(1) in the lung. Quantitative parameters, such as the decrease of T(1) after switching the breathing gas, were more suited to oxygen transfer quantification than to relative signal enhancement. The mean T(1) value during inhalation of room air (T(1,room)) for the volunteers was 1260 msec. This value decreased by about 10% after switching the breathing gas to carbogen. For the patients, the mean T(1,room) value was 1182 msec, which decreased by about 7% when breathing carbogen. The parameter maps generated using the proposed hybrid method deviated, on average, only about 1% from the T(1)-maps. CONCLUSION: For the purpose of intersubject comparison, OE lung imaging should be performed quantitatively. The proposed hybrid technique produced reliable quantitative results in a short amount of time and, therefore, is suited for clinical use.     

Acute cerebral ischemia in rats studied by Carr-Purcell spin-echo magnetic resonance imaging: assessment of blood oxygenation level-dependent and tissue effects on the transverse relaxation.

Acute cerebral ischemia has been shown to be associated with an enhanced transverse relaxation rate in rat brain parenchyma, chiefly due to the blood oxygenation level-dependent (BOLD) effect. In this study, Carr-Purcell R(2) (CP R(2)), acquired both with short and long time intervals between centers of adiabatic pi-pulses (tau(CP)), was used to assess the contributions of BOLD and tissue effects to the transverse relaxation in two brain ischemia models of rat at 4.7 T. R(1rho) and diffusion MR images were also acquired in the same animals. During the first minutes of global ischemia, the long tau(CP) R(2) in brain parenchyma increased, whereas the short tau(CP) R(2) was unchanged. Based on the simulations, and using constraints of intravascular BOLD effect on parenchymal R(2), the former observation was ascribed to be due to susceptibility changes arising in the extravascular compartment. R(1rho) declined almost immediately after the onset of focal cerebral ischemia, and further declined during the evolution of ischemic damage. Interestingly, short tau(CP) CP R(2) started to decline after some 20 min of focal ischemia and declined over a time course similar to that of R(1rho), indicating that it may be an MRI marker for irreversible tissue changes in cerebral ischemia. The present results show that CP R(2) MRI can reveal both tissue- and blood-derived contrast changes in acute cerebral ischemia.     

地震伤磁共振检查扫描方案的探讨

目的：探讨地震伤MRI检查的扫描时间、序列和成像平面在快速显示病变中的作用,提出地震伤MRI检查的合理扫描方案。方法：用1．5TMR根据临床要求和伤情,对20例汶川地震伤员分别行横轴位和冠状位的T1WI、T2WI及T2WI＋FS扫描,评价显示病变的效果和成像时间。结果：T2WI＋Fs序列对病变的显示范围优于T2WI,但显示解剖结构比T2WI差。轴位显示病变与周围组织清晰,但成像时间成倍增加;冠状位能在很短时间内完成较大范围扫描。结论：T1WI、T2WI及T2WI＋FS为必须扫描序列,轴位和冠状位为必须基本扫描平面;增加重BWI水成像和GRE快速序列,以显示继发脏器损伤。