Rahel Heule

Triple echo steady‐state (TESS) relaxometry

Rapid imaging techniques have attracted increased interest for relaxometry, but none are perfect: they are prone to static (B0) and transmit (B1) field heterogeneities, and commonly biased by T2/T1. The purpose of this study is the development of a rapid T1 and T2 relaxometry method that is completely (T2) or partly (T1) bias‐free.

Local quantum control of Heisenberg spin chains

Motivated by some recent results of quantum control theory, we discuss the feasibility of local operator control in arrays of interacting qubits modeled as isotropic Heisenberg spin chains. Acting on one of the end spins, we aim at finding piecewise-constant control pulses that lead to optimal fidelities for a chosen set of quantum gates. We analyze the robustness of the obtained results for the gate fidelities to random errors in the control fields, finding that with faster switching between piecewise-constant controls the system is less susceptible to these errors. The observed behavior falls into a generic class of physical phenomena that are related to a competition between resonance- and relaxation-type behavior, exemplified by motional narrowing in NMR experiments. Finally, we discuss how the obtained optimal gate fidelities are altered when the corresponding rapidly varying piecewise-constant control fields are smoothened through spectral filtering.

Triple-echo steady-state T2 relaxometry of the human brain at high to ultra-high fields

Quantitative MRI techniques, such as T2 relaxometry, have demonstrated the potential to detect changes in the tissue microstructure of the human brain with higher specificity to the underlying pathology than in conventional morphological imaging. At high to ultra‐high field strengths, quantitative MR‐based tissue characterization benefits from the higher signal‐to‐noise ratio traded for either improved resolution or reduced scan time, but is impaired by severe static (B0) and transmit (B1) field heterogeneities. The objective of this study was to derive a robust relaxometry technique for fast T2 mapping of the human brain at high to ultra‐high fields, which is highly insensitive to B0 and B1 field variations. The proposed method relies on a recently presented three‐dimensional (3D) triple‐echo steady‐state (TESS) imaging approach that has proven to be suitable for fast intrinsically B1‐insensitive T2 relaxometry of rigid targets. In this work, 3D TESS imaging is adapted for rapid high‐ to ultra‐high‐field two‐dimensional (2D) acquisitions. The achieved short scan times of 2D TESS measurements reduce motion sensitivity and make TESS‐based T2 quantification feasible in the brain. After validation in vitro and in vivo at 3 T, T2 maps of the human brain were obtained at 7 and 9.4 T. Excellent agreement between TESS‐based T2 measurements and reference single‐echo spin‐echo data was found in vitro and in vivo at 3 T, and T2 relaxometry based on TESS imaging was proven to be feasible and reliable in the human brain at 7 and 9.4 T. Although prominent B0 and B1 field variations occur at ultra‐high fields, the T2 maps obtained show no B0‐ or B1‐related degradations. In conclusion, as a result of the observed robustness, TESS T2 may emerge as a valuable measure for the early diagnosis and progression monitoring of brain diseases in high‐resolution 2D acquisitions at high to ultra‐high fields. Copyright

Rapid estimation of cartilage T2 with reduced T1 sensitivity using double echo steady state imaging

In principle, double echo steady state (DESS) offers morphological and quantitative T2 imaging of cartilage within one single scan. However, accurate T2 estimation is hampered by its prominent T1 dependency in the limit of low flip angles, generally used to image cartilage morphology, as for the osteoarthritis initiative. A new postprocessing approach is introduced to overcome this T1‐related bias for rapid DESS‐based T2 quantification in the low flip angle regime.

Variable flip angle T1 mapping in the human brain with reduced t2 sensitivity using fast radiofrequency‐spoiled gradient echo imaging

Variable flip angle (VFA) T1 quantification using three‐dimensional (3D) radiofrequency (RF) spoiled gradient echo imaging offers the acquisition of whole‐brain T1 maps in clinically acceptable times. However, conventional VFA T1 relaxometry is biased by incomplete spoiling (i.e., residual T2 dependency). A new postprocessing approach is proposed to overcome this T2‐related bias.

High-Resolution Axonal Bundle (Fascicle) Assessment and Triple-Echo Steady-State T2 Mapping of the Median Nerve at 7 T: Preliminary Experience.

ObjectivesThe aims of this preliminary study were to determine the number of axonal bundles (fascicles) in the median nerve,1 using a high-resolution, proton density (PD)–turbo spin echo (TSE) fat suppression sequence, and to determine normative T2 values, measured by triple-echo steady state, of the median nerve in healthy volunteers and in patients with idiopathic carpal tunnel syndrome (CTS), at 7 T.2 Materials and MethodsThis prospective study was approved by the local ethics committee and conducted between March 2014 and January 2015. All study participants gave written informed consent. Six healthy volunteers (30 ± 12 years) and 5 patients with CTS (44 ± 16 years) were included. Measurements were performed on both wrists in all volunteers and on the affected wrist in patients (3 right, 2 left). Based on 5-point scales, 2 readers assessed image quality (1, very poor; 5, very good) and the presence of artifacts that might have a possible influence on fascicle determination (1, severe artifacts; 5, no artifacts) and counted the number of fascicles independently on the PD-TSE sequences. Furthermore, T2 values by region of interest analysis were assessed. Student t tests, a hierarchic linear model, and intraclass correlation coefficients (ICCs) were used for statistical analysis. ResultsProton density-TSE image quality and artifacts revealed a median of 5 in healthy volunteers and 4 in patients with CTS for both readers. Fascicle count of the median nerve ranged from 13 to 23 in all subjects, with an ICC of 0.87 (95% confidence interval [CI], 0.67–0.95). T2 values were significantly higher (P = 0.023) in patients (24.27 ± 0.97 milliseconds [95% CI, 22.19–26.38]) compared with healthy volunteers (21.01 ± 0.65 milliseconds [95% CI, 19.61–22.41]). The ICC for all T2 values was 0.97 (95% CI, 0.96–0.98). ConclusionsThis study shows the possibility of fascicle determination of the median nerve in healthy volunteers and patients with CTS (although probably less accurately) with high-resolution 7 T magnetic resonance imaging, as well as significantly higher T2 values in patients with CTS, which seems to be associated with pathophysiological nerve changes.

Simultaneous multislice triple-echo steady-state (SMS-TESS) T1, T2, PD, and off-resonance mapping in the human brain

To investigate the ability of simultaneous multislice triple‐echo steady‐state (SMS‐TESS) imaging to provide quantitative maps of multiple tissue parameters, i.e., longitudinal and transverse relaxation times (T1 and T2), proton density (PD), and off‐resonance (ΔB0), in the human brain at 3T from a single scan.

Rapid and robust variable flip angle T1 mapping using interleaved two-dimensional multislice spoiled gradient echo imaging

Conventional T1 mapping using three‐dimensional (3D) radiofrequency (RF) spoiled gradient echo (SPGR) imaging with short repetition times (TR) is adversely affected by incomplete spoiling (i.e. residual T2 dependency). In this work, an optimized interleaved 2D multislice SPGR sequence scheme and an adapted postprocessing procedure are evaluated for highly T2‐insensitive T1 quantification of human brain tissues.

Snapshot whole‐brain T1 relaxometry using steady‐state prepared spiral multislice variable flip angle imaging

Variable flip angle (VFA) imaging is widely used for whole‐brain T1 quantification. Because of the requirement to acquire at least two sets of MR images at different flip angles, VFA relaxometry is relatively slow. Here, whole‐brain VFA T1 mapping at 1.5 T is accelerated by using efficient spiral non‐Cartesian imaging

T 2 mapping of cartilage with triple-echo steady state MR sequence

The goal of this work was to analyze the clinical and technical usability of recently introduced steady state free precession T2 mapping technique. The TESS-T2 values were compared to conventionally used multi-echo spin-echo sequence (usually referred to as CPMG). The results showed that although T2 values determined by TESS are lower than those of CPMG, they are highly correlated between two methods. Both, clinical and experimental studies might benefit from the speed of the 3D-TESS. The results of this study showed the clinical usability of a 3D-TESS sequence for T2 mapping of human articular knee cartilage. 3D-TESS provides highly correlated T2-values to conventional CMPG sequence with some additional benefits, such as dramatic shortening of acquisition time andfair insensitivity to B1 and B0 variations. Also, T2 values acquired with 3D-TESS can differentiate between reference and impaired cartilage.