Purpose

There are many T1 mapping methods available, each of them validated in phantoms and reporting excellent agreement with literature. However, values in literature vary greatly, with T1 in white matter ranging from 690 to 1100 ms at 3 Tesla. This brings into question the accuracy of one of the most fundamental measurements in quantitative MRI. Our goal was to explain these variations and look into ways of mitigating them.



Theory and Methods

We evaluated the three most common T1 mapping methods (inversion recovery, Look-Locker, and variable flip angle) through Bloch simulations, a white matter phantom and the brains of 10 healthy subjects (single-slice). We pooled the T1 histograms of the subjects to determine whether there is a sequence-dependent bias and whether it is reproducible across subjects.



Results

We found good agreement between the three methods in phantoms, but poor agreement in vivo, with the white matter T1 histogram peak in healthy subjects varying by more than 30% depending on the method used. We also found that the pooled brain histograms displayed three distinct white matter peaks, with Look-Locker consistently underestimating, and variable flip angle overestimating the inversion recovery T1 values. The Bloch simulations indicated that incomplete spoiling and inaccurate B1 mapping could account for the observed differences.



Conclusion

We conclude that the three most common T1 mapping protocols produce stable T1 values in phantoms, but not in vivo. To improve the accuracy of T1 mapping, we recommend that sites perform in vivo validation of their T1 mapping method against the inversion recovery reference method, as the first step toward developing a robust calibration scheme. Magn Reson Med 73:514–522, 2015. © 2014 Wiley Periodicals, Inc.

On the accuracy of T1 mapping: Searching for common ground

Objectives: The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is considered to have potential neuro-invasiveness that might lead to acute brain disorders or contribute to respiratory distress in patients with coronavirus disease 2019 (COVID-19). This study investigates the occurrence of structural brain abnormalities in non-survivors of COVID-19 in a virtopsy framework. Methods: In this prospective, monocentric, case series study, consecutive patients who fulfilled the following inclusion criteria benefited from an early postmortem structural brain MRI: death Results: Among the 62 patients who died from COVID-19 from 31/03/2020 to 24/04/2020 at our institution, 19 decedents fulfilled the inclusion criteria. Parenchymal brain abnormalities were observed in 4 decedents: subcortical micro- and macro-bleeds (2 decedents), cortico-subcortical edematous changes evocative of posterior reversible encephalopathy syndrome (PRES, one decedent), and nonspecific deep white matter changes (one decedent). Asymmetric olfactory bulbs were found in 4 other decedents without downstream olfactory tract abnormalities. No brainstem MRI signal abnormality was observed. Conclusions: Postmortem brain MRI demonstrates hemorrhagic and PRES-related brain lesions in non-survivors of COVID-19. SARS-CoV-2-related olfactory impairment seems to be limited to olfactory bulbs. Brainstem MRI findings do not support a brain-related contribution to respiratory distress in COVID-19.

/pdf/early-postmortem-brain-mri-findings-in-covid-19-non-2vmgt8jaia.pdf

Early postmortem brain MRI findings in COVID-19 non-survivors.

In this article, a robust methodology for in vivo T(1) mapping is presented. The approach combines a gold standard scanning procedure with a novel fitting procedure. Fitting complex data to a five-parameter model ensures accuracy and precision of the T(1) estimation. A reduced-dimension nonlinear least squares method is proposed. This method turns the complicated multi-parameter minimization into a straightforward one-dimensional search. As the range of possible T(1) values is known, a global grid search can be used, ensuring that a global optimal solution is found. When only magnitude data are available, the algorithm is adapted to concurrently restore polarity. The performance of the new algorithm is demonstrated in simulations and phantom experiments. The new algorithm is as accurate and precise as the conventionally used Levenberg-Marquardt algorithm but much faster. This gain in speed makes the use of the five-parameter model viable. In addition, the new algorithm does not require initialization of the search parameters. Finally, the methodology is applied in vivo to conventional brain imaging and to skin imaging. T(1) values are estimated for white matter and gray matter at 1.5 T and for dermis, hypodermis, and muscle at 1.5 T, 3 T, and 7 T.

/pdf/a-robust-methodology-for-in-vivo-t1-mapping-1473gdfs8h.pdf

A robust methodology for in vivo T1 mapping.

Rapid measurements of in vivo proton spin-lattice relaxation times (T1) in human tissues were performed by magnetic resonance imaging in a 1.5 T whole-body super-conducting MR scanner. The measurements employ serial TurboFLASH imaging (Snapshot-FLASH) with scan times for a single experiment below 4 s. Using centric phase encoding order, an appropriate fitling of the T1-parameter from images with minimum motion artifacts is possible. Comparative T1-determination with a multipoint inversion recovery and spin-echo technique was performed on phantoms containing Gd-DTPA solutions with different T1-values. We found a maximum deviation of 3.3% for T1 < 1100 ms and of 12.1% for 1100 ms < T1 < 1700 ms from the results obtained by the IR technique. In vivo measurements of T1-relaxation times were performed in white and grey matter, cerebrospinal fluid, kidney, liver, spleen, muscle, and bone marrow, and yielded values that are in good agreement with reported data.

Spin-lattice relaxation time measurement by means of a TurboFLASH technique

Objective: Nanosized materials of gadolinium oxide can provide high-contrast enhancement in magnetic resonance imaging (MRI). The objective of the present study was to investigate proton relaxation ...

S.M. Tung

Papers

Temperature dependence of proton relaxation times in vitro.