MR lung imaging at 0.2 T with T1-weighted true FISP: native and oxygen-enhanced

An inversion recovery true fast imaging with steady precession (FISP) pulse sequence was developed to carry out fast imaging of the lungs at 0.2 T. Using this sequence, oxygen-enhanced magnetic resonance (MR) lung imaging was performed on healthy volunteers. The lungs showed signal enhancement (11.7% +/- 3.8%) when breathing 100% oxygen. Using inversion recovery, true FISP at low field may prove promising for MR lung imaging.     

Inversion recovery at 7 T in the human myocardium: Measurement of T1, inversion efficiency and B1+

At clinical MRI field strengths (1.5 and 3 T), quantitative maps of the longitudinal relaxation time T₁ of the myocardium reveal diseased tissue without requiring contrast agents. Cardiac T₁ maps can be measured by Look‐Locker inversion recovery sequences such as ShMOLLI at 1.5 and 3 T. Cardiovascular MRI at a field strength of 7 T has recently become feasible, but doubts have remained as to whether magnetization inversion is possible in the heart due to subject heating and technical limitations. This work extends the repertoire of 7 T cardiovascular MRI by implementing an adiabatic inversion pulse optimized for use in the heart at 7 T. A “ShMOLLI+IE” adaptation of the ShMOLLI pulse sequence has been introduced together with new postprocessing that accounts for the possibility of incomplete magnetization inversion. These methods were validated in phantoms and then used in a study of six healthy volunteers to determine the degree of magnetization inversion and the T₁ of normal myocardium at 7 T within a 22‐heartbeat breathhold. Using a scanner with 16 × 1 kW radiofrequency outputs, inversion efficiencies ranging from ?0.79 to ?0.83 (intrasegment means; perfect 180° would give ?1) were attainable across the myocardium. The myocardial T₁ was 1925 ± 48 ms (mean ± standard deviation). Magn Reson Med, 70:1038–1046, 2013. © 2012 Wiley Periodicals, Inc.     

Optimized double inversion recovery for reduction of T1 weighting in fluid‐attenuated inversion recovery

Fluid‐attenuated inversion recovery (FLAIR) is a routinely used technique in clinical practice to detect long T₂ lesions by suppressing the cerebrospinal fluid. Concerns remain, however, that the inversion pulse in FLAIR imparts T₁ weighting that can decrease the detectability and mischaracterize some lesions. Hence, FLAIR is usually acquired in conjunction with a standard T₂ to guard against these concerns. Recently, double inversion recovery (DIR) preparations have highlighted certain types of lesions by suppressing both cerebrospinal fluid and white matter but produce even stronger T₁ contrast than FLAIR. This work shows that the inversion times in a DIR sequence can be optimized to minimize unwanted T₁ weighting, enabling the acquisition of cerebrospinal fluid‐suppressed images with pure T₂ weighting. This technique is referred to as T₁‐nulled DIR. The theory to determine the optimized inversion times is discussed and the results are shown by simulations, normal volunteer studies, and multiple sclerosis patient studies. T₁‐nulled DIR provides equivalent or superior contrast between gray and white matters as well as white matter and multiple sclerosis lesion at the same repetition time. Multiple sclerosis lesions appeared sharper on T₁‐nulled DIR compared to FLAIR. T₁‐nulled DIR has the potential to replace the combination of standard T₂ and FLAIR acquisitions in many clinical protocols. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

In vivo blood T1 measurements at 1.5 T, 3 T,and 7 T

The longitudinal relaxation time of blood is a crucial parameter for quantification of cerebral blood flow by arterial spin labeling and is one of the main determinants of the signal‐to‐noise ratio of the resulting perfusion maps. Whereas at low and medium magnetic field strengths (B₀), its in vivo value is well established; at ultra‐high field, this is still uncertain. In this study, longitudinal relaxation time of blood in the sagittal sinus was measured at 1.5 T, 3 T, and 7 T. A nonselective inversion pulse preceding a Look‐Locker echo planar imaging sequence was performed to obtain the inversion recovery curve of venous blood. The results showed that longitudinal relaxation time of blood at 7 T was ～ 2.1 s which translates to an anticipated 33% gain in the signal‐to‐noise ratio in arterial spin labeling experiments due to T₁ relaxation alone compared with 3 T. In addition, the linear relationship between longitudinal relaxation time of blood and B₀ was confirmed. Magn Reson Med, 70:1082–1086, 2013. © 2012 Wiley Periodicals, Inc.     

Phase‐sensitive inversion recovery for myocardial T1 mapping with motion correction and parametric fitting

The assessment of myocardial fibrosis and extracellular volume requires accurate estimation of myocardial T₁s. While image acquisition using the modified Look‐Locker inversion recovery technique is clinically feasible for myocardial T₁ mapping, respiratory motion can limit its applicability. Moreover, the conventional T₁ fitting approach using the magnitude inversion recovery images can lead to less stable T₁ estimates and increased computational cost. In this article, we propose a novel T₁ mapping scheme that is based on phase‐sensitive image reconstruction and the restoration of polarity of the MR signal after inversion. The motion correction is achieved by registering the reconstructed images after background phase removal. The restored signal polarity of the inversion recovery signal helps the T₁ fitting resulting in improved quality of the T₁ map and reducing the computational cost. Quantitative validation on a data cohort of 45 patients proves the robustness of the proposed method against varying image contrast. Compared to the magnitude T₁ fitting, the proposed phase‐sensitive method leads to less fluctuation in T₁ estimates. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Determining the longitudinal relaxation time (T1) of blood at 3.0 Tesla.

It is important to determine the longitudinal relaxation time of blood for black blood imaging, as well as for quantifying blood flow by arterial spin labeling (ASL). In this study a circulation system was used to measure blood T1 under physiological conditions at the new clinical field strength of 3.0T. It was found that 1/T1 in s(-1) was linearly dependent (P < 0.05) on hematocrit (Hct) within a normal range of 0.38-0.46. The relationships were 1/T1 = (0.52 +/- 0.15). Hct + (0.38 +/- 0.06) and 1/T1 = (0.83 +/- 0.07). Hct + (0.28 +/- 0.03) for arterial (oxygenation = 92% +/- 7%) and venous blood (69% +/- 8%), respectively, which led to estimated T1 values of 1664 +/- 14 ms (arterial) and 1584 +/- 5 ms (venous) at a typical human Hct of 0.42. The temperature dependencies of blood T1 were 22.3 +/- 0.6 ms/ degrees C and 19.8 +/- 0.8 ms/ degrees C for Hct values of 0.42 and 0.38, respectively. When a head coil transmit/receive setup was used, radiation damping caused a slight reduction (19 ms) of the measured T1 values.     

Effect of blood flow on double inversion recovery vessel wall MRI of the peripheral arteries: Quantitation with T2 mapping and comparison with flow‐insensitive T2‐prepared inversion recovery imaging

Blood suppression in the lower extremities using flow‐reliant methods such as double inversion recovery may be problematic due to slow blood flow. T₂ mapping using fast spin echo (FSE) acquisition was utilized to quantitate the effectiveness of double inversion recovery blood suppression in 13 subjects and showed that 25 ± 12% of perceived vessel wall pixels in the popliteal arteries contained artifactual blood signal. To overcome this problem, a flow‐insensitive T₂‐prepared inversion recovery sequence was implemented and optimal timing parameters were calculated for FSE acquisition. Black blood vessel wall imaging of the popliteal and femoral arteries was performed using two‐dimensional T₂‐prepared inversion recovery‐FSE in the same 13 subjects. Comparison with two‐dimensional double inversion recovery‐FSE showed that T₂‐prepared inversion recovery‐FSE reduced wall‐mimicking blood artifacts that inflated double inversion recovery‐FSE vessel wall area measurements in the popliteal artery. Magn Reson Med 63:736–744, 2010. © 2010 Wiley‐Liss, Inc.     

Fast T1 mapping determined using incomplete inversion recovery look-locker 3D balanced SSFP acquisition and a simple two-parameter model fit

Purpose:

To evaluate a fast T1 mapping technique using incomplete inversion recovery 3D balanced steady‐state free precession acquisition along with a two‐parameter model fit.

Materials and Methods:

Using Bloch simulations, we explored the two‐parameter model fit for data acquired using such an acquisition scheme. The parameter space over which the fit holds good was determined through simulations. A linear correction was derived for the R1* (1/T1*) values so determined. Two phantoms and six volunteers were scanned using the described technique. Comparison scans using full recovery as well as gold standard inversion recovery spin echo were also performed.

Results:

The two‐parameter fit works exceedingly well over a large parameter space. T1 values in the phantoms showed an error of 4.9% and 39% before correction and 0.9% and 1.6% after correction. For the six volunteers, error in T1 value was 5.3% for white matter (WM) and 2.4% for gray matter (GM) after correction, while it was 11.2% and 18.2% before correction.

Conclusion:

The work presented here allows for T1 map determination with higher resolution and shorter acquisition time than previously possible. The technique is especially well suited for GM/WM T1 mapping. J. Magn. Reson. Imaging 2012;35:1437–1444. © 2012 Wiley Periodicals Inc.     

Measurement of deep gray matter perfusion using a segmented true–fast imaging with steady‐state precession (True‐FISP) arterial spin‐labeling (ASL) method at 3T

Purpose

To study the feasibility of using the MRI technique of segmented true–fast imaging with steady‐state precession arterial spin‐labeling (True‐FISP ASL) for the noninvasive measurement and quantification of local perfusion in cerebral deep gray matter at 3T.

Materials and Methods

A flow‐sensitive alternating inversion‐recovery (FAIR) ASL perfusion preparation was used in which the echo‐planar imaging (EPI) readout was replaced with a segmented True‐FISP data acquisition strategy. The absolute perfusion for six selected regions of deep gray matter (left and right thalamus, putamen, and caudate) were calculated in 11 healthy human subjects (six male, five female; mean age = 35.5 years ± 9.9).

Results

Preliminary measurements of the average absolute perfusion values at the six selected regions of deep gray matter are in agreement with published values for mean absolute cerebral blood flow (CBF) baselines acquired from healthy volunteers using positron emission tomography (PET).

Conclusion

Segmented True‐FISP ASL is a practical and quantitative technique suitable to measure local tissue perfusion in cerebral deep gray matter at a high spatial resolution without the susceptibility artifacts commonly associated with EPI‐based methods of ASL. J. Magn. Reson. Imaging 2009;29:1425–1431. © 2009 Wiley‐Liss, Inc.     

Accurate T1 determination from inversion recovery images: Application to human brain at 4 Tesla

It is well known that the signal polarity in inversion-recovery (IR) images changes with inversion time, complicating the determination of T₁ To avoid this problem, a simple subtraction method is implemented. In this method, k-space data of the longest inversion time are subtracted from the corresponding data of each inversion time. This subtraction yields IR images of same polarity, making it straightforward to derive T₁ using a standard fitting routine. Phantom T₁ studies with IR Turbo-FLASH images demonstrate that this technique is robust and accurate. Four Tesla T₁ values of the human brain were also determined by this method to demonstrate its in vivo utility.     

T(1) quantification with inversion recovery TrueFISP.

A snapshot FLASH sequence can be used to acquire the time course of longitudinal magnetization during its recovery after a single inversion pulse. However, excitation pulses disturb the exponential recovery of longitudinal magnetization and may produce systematic errors in T(1) estimations. In this context the possibility of using the TrueFISP sequence to detect the recovery of longitudinal magnetization for quantitative T(1) measurements was examined. Experiments were performed on different Gd-doped water phantoms and on humans. T(1) values derived from inversion recovery TrueFISP were in excellent agreement with the single-point method even for flip angles up to 50 degrees. In terms of T(1) accuracy and SNR, the proposed method seems to be superior to the conventional inversion recovery snapshot FLASH technique. Magn Reson Med 45:720-723, 2001.     

In vitro investigation of poor cerebrospinal fluid suppression on fluid-attenuated inversion recovery images in the presence of a gadolinium-based contrast agent.

Cerebrospinal fluid (CSF) enhancement on fluid-attenuated inversion recovery (FLAIR) images obtained post-gadolinium (Gd)-based agent injection is described in stroke and multiple sclerosis. Blood brain barrier (BBB) disruption with contrast agent extravasation into CSF shortens T(1) relaxation times, reducing fluid suppression. Reduced fluid suppression on FLAIR images was investigated in vitro in the presence of escalating gadopentetate dimeglumine (Gd-DTPA) concentrations mixed with artificial CSF. Low Gd-DTPA concentrations impair fluid suppression of FLAIR imaging in association with progressively reduced T(1) values. At higher concentrations, the prevalent T(2) shortening effect can explain signal intensity (SI) reduction. Post-Gd FLAIR may be useful in detecting subtle BBB leakage.     

Fast measurement of blood T1 in the human jugular vein at 3 Tesla

Current T₁ values for blood at 3T largely came from in vitro studies on animal blood or freshly drawn human blood. Measurement of blood T₁ in vivo could provide more specific information, e.g., for individuals with abnormal blood composition. Here, blood T₁ at 3T was measured rapidly (<1 min) in the internal jugular vein using a fast inversion‐recovery technique in which multiple inversion time can be acquired rapidly due to constant refreshing of blood. Multishot EPI acquisition with flow compensation yielded high resolution images with minimum partial volume effect. Results showed T₁ = 1852 ± 104 msec among 24 healthy adults, a value higher than for bovine blood phantoms (1584 msec at Hct of 42%). A second finding was that of a significant difference (P < 0.01) between men and women, namely T₁ = 1780 ± 89 msec (n = 12) and T₁ = 1924 ± 58 msec (n = 12), respectively. This difference in normal subjects is tentatively explained by the difference in Hct between genders. Interestingly, however, studies done on sickle cell anemia patients with much lower Hct (23 ± 3%, n = 10) revealed similar venous blood T₁ = 1924 ± 82 msec, indicating other possible physical influences affecting blood T₁. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.