Reduced RF power without blurring: correcting for modulation of refocusing flip angle in FSE sequences.

To compare conspicuity and detection rate of hypointense lesions on T1-weighted (T1w) gradient echo (GRE) sequences with low and high flip angles (FA) in hepatocyte phase magnetic resonance imaging (MRI) using gadoxetate disodium. This Health Insurance Portability and Accountability Act (HIPAA)-compliant study was Institutional Review Board (IRB)-approved. The study population consisted of patients with hypointense liver lesions undergoing MRI with gadoxetate disodium, with hepatocyte-phase fat suppressed 3D T1w GRE sequences at both low (10-12°) and high (30-35°) FA. Contrast-to-noise ratios (CNRs) were calculated for liver parenchyma vs. large lesions and common bile duct (CBD) vs. liver. Three radiologists each assigned a conspicuity score (CS) for each lesion detected at low or high FA. Paired Student's t-tests compared the lesion detection (LD) rate using only the hepatocyte phase data set compared with the entire MRI examination, and CS for low and high FA. In all, 57 large and 70 small lesions were identified in 18 patients. Average LD and CS were significantly greater at high FA versus low FA overall (LD 89.0% vs. 79.5%; CS 2.8 vs. 2.2; P < 0.05) and for small lesions (81.4% vs. 65.7%; 2.5 vs. 1.8; P < 0.05). Average liver-to-lesion CNR for large lesions and CBD-to-liver CNR was significantly greater at high FA (P < 0.05). Increasing the FA in hepatocyte phase MRI with gadoxetate disodium improves hypointense lesion detection and conspicuity, particularly for small lesions.

To compare the conspicuity of hypointense hepatocellular nodules in patients with chronic liver disease on hepatobiliary phase (HP) of gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd-EOB-DTPA)-enhanced magnetic resonance imaging (MRI) acquired with low to high flip angles (FAs). A total of 95 patients with chronic liver disease who underwent Gd-EOB-DTPA-enhanced MRI were included. HP images were obtained at 20 minutes, with 15°, 20°, and 30° FAs. For the detected hepatocellular nodule, liver-to-lesion contrast-to-phantom ratios (CPR) and lesion conspicuity (LCS) were assessed. In all examinations, 96 hepatocellular nodules showing hypointensity on HP were identified. These lesions included 39 hypovascular nodules and 57 hypervascular nodules. Mean CPR and LCS showed the highest value on the 30° FA, followed by 20° and 15° FAs. CPR and LCS of 15° FA were significantly lower than those of 20° and 30° FAs (P < 0.001 to P = 0.007). CPR of 30° FA for hypervascular nodules was significantly greater than that of 20° FA (P < 0.001). In the evaluation of hypointense hepatocellular nodules on HP of Gd-EOB-DTPA-enhanced MRI, higher FA such as 30° should be used rather than low FA such as 15°.

Evaluation of the inner ear by 3D fast asymmetric spin echo (FASE) MR imaging: phantom and volunteer studies

To compare the accuracy and precision of four different T1 estimation algorithms for modified Look-Locker inversion recovery (MOLLI). Four T1 estimation algorithms, including the original fit, inversion group (IG) fit, instantaneous signal loss simulation (InSiL), and Bloch equation simulation with slice profile correction (BLESSPC) were studied. T1 estimation accuracy, precision, reproducibility, and sensitivity to heart rate (HR), flip angle (FA), and acquisition scheme (AcS) variations were compared in simulation, phantom, and volunteer studies. T1 estimation accuracy of IG (-2.4% ± 3.9%) and original fit (-3.2% ± 1.4%) were worse than BLESSPC (0.2% ± 1.5%) and InSiL (-0.7% ± 2.1%). The original fit had the best precision for T1 from 409-1884 ms for the same FA (0.67% ± 0.16% versus 0.90% ± 0.23% using IG, 0.78% ± 0.11% using InSiL, 0.77% ± 0.12% using BLESSPC). BLESSPC generated the most consistent in vivo T1 values over different FAs and AcS, and the T1 estimation reproducibility was similar (P > 0.3) among the four methods when FA = 35°. When using FA = 50°, the reproducibility was significantly improved only when using BLESSPC (1.6% ± 0.9 versus 2.6% ± 1.9%, P < 0.05). BLESSPC has superior accuracy and is the least sensitive to FA, HR, and AcS variations. T1 estimation using BLESSPC and FA = 50° is superior to conventional MOLLI with FA = 35° in accuracy and precision. Further clinical studies in varying pathological conditions are warranted to confirm our findings. Magn Reson Med 78:1746-1756, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

In recent years a paradigm shift in therapy evaluations for oncology is apparent. Focusing on the morphological changes with the variety of imaging modalities is not sufficient to assess therapy efficacy during treatment. &#13;\n&#13;\nIn the first part of this thesis the changes of relaxation times were quantified in ex vivo porcine recta during formaldehyde fixation and in human specimen. Chapter 2 concludes that T1 of the circular muscle, mucosa and submucosa, and T2 of the circular muscle decreased. Consequently, fixation, prior to imaging, progressively decreases image contrast. The human specimens were therefore imaged during the first few hours of fixation, in search for a suitable diagnostic marker. However, no conclusive parameter resulted as a significant diagnostic parameter for therapy evaluation.&#13;\n &#13;\nFirst step towards bringing metabolic imaging to patients is good magnetic field (B0) uniformity. Improvements of the B0-field homogeneity by simulating a local array of shim coils on liver MRI during different respiratory states are investigated in chapter 3. From simulations it is apparent that the increased degree of freedom improves homogeneity to a greater extent (10%) than using conventional shim hardware. Shimming B0 dependent on respiratory states reduces the effects of the respiratory motion with improvements ranging from 21 to 44 %. &#13;\n&#13;\nChapter 4 facilitates the next step by integrating a 31P whole-body birdcage coil in 7T MR system, to enable 31P MRS over large fields of views. The design allows use of rectangular RF-pulses which decreases overall SAR compared to adiabatic RF-pulses while maintaining flip-angle homogeneity. It permits exchange in SNR for an increase in resolution and/or MR-weighting in 31P MRSI. The number of RF-pulses/dt can be increased as is demonstrated by fast 3D CSI of the liver with a short TR and low flip-angle excitation to acquire optimal SNR/dt. Furthermore, T2 of PCr in the gluteal muscle was quantified using a MESING protocol, something previously not possible large volumes. &#13;\n&#13;\nInterpretation of accompanied data required development of a software tool, CSIgui as explained in chapter 5. The large data sets have a high number of dimensionalities and are therefore not handled well by current software solutions. The tool is compatible with conventional MRS and raw file types. Data can be processed using a variety of MR algorithms and saved for analysis. &#13;\n&#13;\nChapter 6 focuses on the feasibility of acquiring 3D CSI of tumor tissue in four lung carcinoma patients using the 31P whole-body birdcage coil in combination with a dual-Rx coil or a Rx array setup. Simulations proved the loss in SNR/dt with imperfect flip-angles is minimal using a short TR low flip angle protocol, eliminating a time-consuming calibration process. However, use of the receiver array favored SNR over a larger FOV compared to the dual coil setup. Detected metabolites were ATP and Pi, but also the phospholipid metabolites including the individual PME, PE and PC, and PDE, GPC and GPE which are not detectable at 3T. 31P MRSI of lung tumors at 7T is feasible and warrants further research as a non-invasive method for therapy evaluation.

This case report shows that paramagnetic rim lesions (PRLs), markers of chronic active lesions in multiple sclerosis, vary in visibility depending on scan-parameters of susceptibility-weighted imaging (SWI). Routine SWI for microbleed detection with low flip angle (FA) failed to depict PRLs, while longer TE and higher FA improved visibility. Phase images consistently visualized PRLs. These findings underscore the need to optimize TE and FA, as suboptimal SWI settings may hinder PRL detection.

ABSTRACTNeuromelanin (NM) magnetic resonance imaging has been used to evaluate the loss of melanized neurons in the substantia nigra, the main characteristic of Parkinson disease, typically by measuring the contrast ratio (CR) between the NM‐rich areas and the cerebral peduncles in the midbrain. Neuromelanin contrast can be generated by employing a 3D gradient‐echo (GRE) sequence with magnetization transfer (MT). In this work, we first evaluated the effect of the MT pulse frequency on the CR and contrast‐to‐noise ratio (CNR), analyzing a large frequency range from 500 to 100K Hz. Secondly, the impact of the 3D GRE sequence flip angle (FA) was evaluated using angles from 5° to 40°. Additionally, images were acquired both with and without MT for each FA, providing an opportunity to examine the effect of MT on tissue proton density, T1, and T2* values. Results showed that the highest CR and CNR were obtained for the lower MT frequencies (500–2000 Hz) and lower FAs (5°–10°). The lower FAs improved the visualization of the neuromelanin‐rich area in the substantia nigra and facilitated delineating its volume. In addition, this work showed that the MT pulse decreased the T1 and PD of the tissue. Furthermore, simulations supported the obtained in vivo results.

To assess the impact of flip angle with gradient sequences on the "magic angle effect". We characterized the magic angle effect in various gradient echo sequences and compared the signal-to-noise ratios present on these sequences with the signal-to-noise ratios of spin echo sequences. Ten normal healthy volunteers were positioned such that the flexor hallucis longus tendon remained at approximately at 55 degrees to the main magnetic field (the magic angle). The tendon was imaged by a conventional spin echo T1- and T2-weighted techniques and by a series of gradient techniques. Gradient sequences were altered by both TE and flip angle. Signal-to-noise measurements were obtained at segments of the flexor hallucis longus tendon demonstrating the magic angle effect to quantify the artifact. Signal-to-noise measurements were compared and statistical analysis performed. Similar measurements were taken of the anterior tibialis tendon as an internal control. We demonstrated the magic angle effect on all the gradient sequences. The intensity of the artifact was affected by both the TE and flip angle. Low TE values and a high flip angle demonstrated the greatest magic angle effect. At TE values less than 30 ms, a high flip angle will markedly increase the magic angle effect.

Our objective was to compare the sensitivity of non-breath-hold T2-weighted fast spin-echo with and without fat suppression, breath-hold T2-weighted fast spin-echo without fat suppression, and spoiled gradient-recalled echo (GRE) MR imaging for detecting hepatic tumors using surgical findings as the standard of reference. Eighteen patients with 36 surgically proven hepatic tumors had non-breath-hold T2-weighted fast spin-echo (6000/117 [TR/effective TE; echo train length, 16) MR imaging with and without fat suppression, breath-hold T2-weighted fast spin-echo MR imaging (2700/105; echo train length, 20), and spoiled GRE images (10.1/1.9; flip angle, 30 degrees) obtained before and after injection of a gadolinium chelate. Images were analyzed separately by two independent readers, with disagreements resolved by consensus reading. Non-breath-hold T2-weighted fast spin-echo MR imaging with and without fat suppression depicted 22 (61%; 95% confidence interval [CI], 43-77%) and 20 (56%; 95% CI, 37-72%) of 36 hepatic tumors, respectively. Breath-hold T2-weighted fast spin-echo imaging allowed detection of 19 (53%; 95% CI, 35-69%) of 36 hepatic tumors. Unenhanced and gadolinium chelate-enhanced spoiled GRE images allowed depiction of 18 (50%; 95% CI, 33-67%) and 29 (81%; 95% CI, 63-91%) of 36 hepatic tumors, respectively. Gadolinium chelate-enhanced spoiled GRE images allowed depiction of significantly more hepatic tumors than any of the other pulse sequences. Gadolinium chelate-enhanced spoiled GRE imaging is more sensitive than T2-weighted MR imaging obtained with a breath-hold or a non-breath-hold technique.

Dynamic susceptibility contrast (DSC) MRI is commonly part of the clinical brain tumor imaging protocol. Usually, a preload of contrast agent is administered to minimize contrast-leakage T1 effects. However, recent studies have indicated that with adaptation of scan parameters (in particular, low flip angle), a preload is not required. For advanced approaches, like MR vascular fingerprinting (MRVF), which is based on combining gradient-echo and spin-echo readouts, excitation flip angles cannot easily be reduced, but postprocessing might alleviate leakage effects. In this study, the aim was to verify whether DSC-MRVF can be performed without preload. A previously established simulation model was used to create a dictionary of DSC-MRVF signals resulting from a double-bolus injection with varying input values for permeability, vessel radius, and relative cerebral blood volume (rCBV). A sensitivity analysis based on a Cramer Rao Lower Bound approach was performed, from which the logarithmic ratio of standard deviations for parameter estimation was derived. A ratio above zero corresponded with a better determination without preload. Two brain tumor patients were included for validation. The results show that the standard deviation for determining permeability was lower without preload, whereas for determining rCBV, the opposite result was found. For vessel radius, the standard deviation was similar with and without preload. The in vivo data reasonably agreed with the simulation results. This work has shown the potential of DSC-MRVF with a single bolus injection for determining vessel radius and permeability, although for rCBV, a preload is still advisable.

To determine the in vitro accuracy, test-retest repeatability, and interplatform reproducibility of T1 quantification protocols used for dynamic contrast-enhanced MRI at 1.5 and 3 T. A T1 phantom with 14 samples was imaged at eight centers with a common inversion-recovery spin-echo (IR-SE) protocol and a variable flip angle (VFA) protocol using seven flip angles, as well as site-specific protocols (VFA with different flip angles, variable repetition time, proton density, and Look-Locker inversion recovery). Factors influencing the accuracy (deviation from reference NMR T1 measurements) and repeatability were assessed using general linear mixed models. Interplatform reproducibility was assessed using coefficients of variation. For the common IR-SE protocol, accuracy (median error across platforms = 1.4-5.5%) was influenced predominantly by T1 sample (P < 10-6 ), whereas test-retest repeatability (median error = 0.2-8.3%) was influenced by the scanner (P < 10-6 ). For the common VFA protocol, accuracy (median error = 5.7-32.2%) was influenced by field strength (P = 0.006), whereas repeatability (median error = 0.7-25.8%) was influenced by the scanner (P < 0.0001). Interplatform reproducibility with the common VFA was lower at 3 T than 1.5 T (P = 0.004), and lower than that of the common IR-SE protocol (coefficient of variation 1.5T: VFA/IR-SE = 11.13%/8.21%, P = 0.028; 3 T: VFA/IR-SE = 22.87%/5.46%, P = 0.001). Among the site-specific protocols, Look-Locker inversion recovery and VFA (2-3 flip angles) protocols showed the best accuracy and repeatability (errors < 15%). The VFA protocols with 2 to 3 flip angles optimized for different applications achieved acceptable balance of extensive spatial coverage, accuracy, and repeatability in T1 quantification (errors < 15%). Further optimization in terms of flip-angle choice for each tissue application, and the use of B1 correction, are needed to improve the robustness of VFA protocols for T1 mapping. Magn Reson Med 79:2564-2575, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

We propose DANTE (Delays Alternating with Nutation for Tailored Excitation) moving fluid attenuation preparation pulse trains, in conjunction with T1 , T2 , and proton-density-weighted fast spin-echo (T1w-TSE, T2w-TSE and PDw-TSE) imaging readout, and three-dimensional fast low flip angle shots (3D-FLASH) T1 -weighted imaging readout to achieve CSF-suppressed high-spatial resolution multicontrast cervical spinal cord images. DANTE pulse trains, consisting of a rapid series of low flip angle radiofrequency pulses interspersed with gradients, were used to substantially attenuate the longitudinal magnetization of flowing spins relative to static tissue/fluid, whose longitudinal magnetization is mostly preserved. We hypothesized that the contrast between spinal cord and cerebrospinal fluid (CSF) could be maximized due to moving CSF signal suppression. We demonstrate that metrics of contrast-to-noise ratio between spinal cord, nerve root, and CSF regions (CNRcord-CSF and CNRnerve-CSF ) are improved by at least a factor of 2 when compared with images acquired with non-prepared approaches and with 2D multiple-echo data image combination (MEDIC) imaging. In addition, we find that sagittal image quality can be significantly improved due to flow suppression effects from the DANTE preparation pluses. DANTE prepared imaging techniques for moving CSF signal attenuation are promising tools for cervical spinal cord imaging.

T2-weighted MRI for the assessment of joint effusion: comparative study of conventional spin-echo and fast spin-echo sequences

The importance to MR image quality of the order of acquisition of different phase-encoded views with sequences that have variable TR and TE has been recently reported. It has been shown that the effective point spread function (PSF) may be manipulated by varying TE or TR, or both, with each phase-encoding step. This paper explores the behavior of the PSF in a variable TE sequence and its dependence on both imaging and tissue parameters. It is shown that the PSF is different for each tissue type and that its effect on tissue contrast is a function of both the shape and size of the structure. The important problem of signal loss from small objects that arises when the effective PSF is broad and the difficulty in detecting this phenomenon in practical MR images is illustrated. It is shown that the PSF can produce significant blurring and loss of object contrast in fast spin-echo images but that this blurring may be not be obvious in practice because the noise is unaffected by the PSF. It is also shown that the signal from small lesions with short T2 can easily be lost through this blurring mechanism. The importance of signal loss from small objects and its implication for the clinical use of such sequences as fast spin-echo or rapid acquisition relaxation-enhanced and echo planar imaging is stressed.

The effects of equilibrium transcytolemmal water exchange on the determination of contrast reagent concentration in vivo