To investigate the sequence-specific impact of
amplitude mapping on the accuracy and precision of permittivity reconstruction at 3T in the pelvic region.
maps obtained with actual flip angle imaging ...(AFI), Bloch-Siegert (BS), and dual refocusing echo acquisition mode (DREAM) sequences, set to a clinically feasible scan time of 5 minutes, were compared in terms of accuracy and precision with electromagnetic and Bloch simulations and MR measurements. Permittivity maps were reconstructed based on these
maps with Helmholtz-based electrical properties tomography. Accuracy and precision in permittivity were assessed. A 2-compartment phantom with properties and size similar to the human pelvis was used for both simulations and measurements. Measurements were also performed on a female volunteer's pelvis.
Accuracy was evaluated with noiseless simulations on the phantom. The maximum
bias relative to the true
distribution was 1% for AFI and BS and 6% to 15% for DREAM. This caused an average permittivity bias relative to the true permittivity of 7% to 20% for AFI and BS and 12% to 35% for DREAM. Precision was assessed in MR experiments. The lowest standard deviation in permittivity, found in the phantom for BS, measured 22.4 relative units and corresponded to a standard deviation in
of 0.2% of the
average value. As regards
precision, in vivo and phantom measurements were comparable.
Our simulation framework quantitatively predicts the different impact of
mapping techniques on permittivity reconstruction and shows high sensitivity of permittivity reconstructions to sequence-specific bias and noise perturbation in the
map. These findings are supported by the experimental results.
Background
Magnetic Resonance Spin TomogrAphy in Time‐domain (MR‐STAT) can reconstruct whole‐brain multi‐parametric quantitative maps (eg, T1, T2) from a 5‐minute MR acquisition. These quantitative ...maps can be leveraged for synthetization of clinical image contrasts.
Purpose
The objective was to assess image quality and overall diagnostic accuracy of synthetic MR‐STAT contrasts compared to conventional contrast‐weighted images.
Study Type
Prospective cross‐sectional clinical trial.
Population
Fifty participants with a median age of 45 years (range: 21–79 years) consisting of 10 healthy participants and 40 patients with neurological diseases (brain tumor, epilepsy, multiple sclerosis or stroke).
Field Strength/Sequence
3T/Conventional contrast‐weighted imaging (T1/T2 weighted, proton density PD weighted, and fluid‐attenuated inversion recovery FLAIR) and a MR‐STAT acquisition (2D Cartesian spoiled gradient echo with varying flip angle preceded by a non‐selective inversion pulse).
Assessment
Quantitative T1, T2, and PD maps were computed from the MR‐STAT acquisition, from which synthetic contrasts were generated. Three neuroradiologists blinded for image type and disease randomly and independently evaluated synthetic and conventional datasets for image quality and diagnostic accuracy, which was assessed by comparison with the clinically confirmed diagnosis.
Statistical Tests
Image quality and consequent acceptability for diagnostic use was assessed with a McNemar's test (one‐sided α = 0.025). Wilcoxon signed rank test with a one‐sided α = 0.025 and a margin of Δ = 0.5 on the 5‐level Likert scale was used to assess non‐inferiority.
Results
All data sets were similar in acceptability for diagnostic use (≥3 Likert‐scale) between techniques (T1w:P = 0.105, PDw:P = 1.000, FLAIR:P = 0.564). However, only the synthetic MR‐STAT T2 weighted images were significantly non‐inferior to their conventional counterpart; all other synthetic datasets were inferior (T1w:P = 0.260, PDw:P = 1.000, FLAIR:P = 1.000). Moreover, true positive/negative rates were similar between techniques (conventional: 88%, MR‐STAT: 84%).
Data Conclusion
MR‐STAT is a quantitative technique that may provide radiologists with clinically useful synthetic contrast images within substantially reduced scan time.
Evidence Level: 1
Technical Efficacy: Stage 2
Purpose
Multi‐transmit MRI systems are typically equipped with dedicated hardware to sample the reflected/lost power in the transmit channels. After extensive calibration, the amplitude and phase of ...the signal at the feed of each array element can be accurately determined. However, determining the phase is more difficult and monitoring errors can lead to a hazardous peak local specific absorption rate (pSAR10g) underestimation. For this purpose, methods were published for online maximum potential pSAR10g estimation without relying on phase monitoring, but these methods produce considerable overestimation. We present a trigonometric maximization method to determine the actual worst‐case pSAR10g without any overestimation.
Theory and Method
The proposed method takes advantage of the sinusoidal relation between the SAR10g in each voxel and the phases of input signals, to return the maximum achievable SAR10g in a few iterations. The method is applied to determine the worst‐case pSAR10g for three multi‐transmit array configurations at 7T: (1) body array with eight fractionated dipoles; (2) head array with eight fractionated dipoles; (3) head array with eight rectangular loops. The obtained worst‐case pSAR10g values are compared with the pSAR10g values determined with a commonly used method and with a more efficient method based on reference‐phases.
Results
For each voxel, the maximum achievable SAR10g is determined in less than 0.1 ms. Compared to the reference‐phases‐based method, the proposed method reduces the mean overestimation of the actual pSAR10g up to 52%, while never underestimating the true pSAR10g.
Conclusion
The proposed method can widely improve the performance of parallel transmission MRI systems without phase monitoring.
Purpose
To investigate the sequence‐specific impact of
amplitude mapping on the accuracy and precision of permittivity reconstruction at 3T in the pelvic region.
Methods
maps obtained with actual ...flip angle imaging (AFI), Bloch–Siegert (BS), and dual refocusing echo acquisition mode (DREAM) sequences, set to a clinically feasible scan time of 5 minutes, were compared in terms of accuracy and precision with electromagnetic and Bloch simulations and MR measurements. Permittivity maps were reconstructed based on these
maps with Helmholtz‐based electrical properties tomography. Accuracy and precision in permittivity were assessed. A 2‐compartment phantom with properties and size similar to the human pelvis was used for both simulations and measurements. Measurements were also performed on a female volunteer’s pelvis.
Results
Accuracy was evaluated with noiseless simulations on the phantom. The maximum
bias relative to the true
distribution was 1% for AFI and BS and 6% to 15% for DREAM. This caused an average permittivity bias relative to the true permittivity of 7% to 20% for AFI and BS and 12% to 35% for DREAM. Precision was assessed in MR experiments. The lowest standard deviation in permittivity, found in the phantom for BS, measured 22.4 relative units and corresponded to a standard deviation in
of 0.2% of the
average value. As regards
precision, in vivo and phantom measurements were comparable.
Conclusions
Our simulation framework quantitatively predicts the different impact of
mapping techniques on permittivity reconstruction and shows high sensitivity of permittivity reconstructions to sequence‐specific bias and noise perturbation in the
map. These findings are supported by the experimental results.
Purpose
To investigate the sequence‐specific impact of B1+ amplitude mapping on the accuracy and precision of permittivity reconstruction at 3T in the pelvic region.
Methods
B1+ maps obtained with ...actual flip angle imaging (AFI), Bloch–Siegert (BS), and dual refocusing echo acquisition mode (DREAM) sequences, set to a clinically feasible scan time of 5 minutes, were compared in terms of accuracy and precision with electromagnetic and Bloch simulations and MR measurements. Permittivity maps were reconstructed based on these B1+ maps with Helmholtz‐based electrical properties tomography. Accuracy and precision in permittivity were assessed. A 2‐compartment phantom with properties and size similar to the human pelvis was used for both simulations and measurements. Measurements were also performed on a female volunteer’s pelvis.
Results
Accuracy was evaluated with noiseless simulations on the phantom. The maximum B1+ bias relative to the true B1+ distribution was 1% for AFI and BS and 6% to 15% for DREAM. This caused an average permittivity bias relative to the true permittivity of 7% to 20% for AFI and BS and 12% to 35% for DREAM. Precision was assessed in MR experiments. The lowest standard deviation in permittivity, found in the phantom for BS, measured 22.4 relative units and corresponded to a standard deviation in B1+ of 0.2% of the B1+ average value. As regards B1+ precision, in vivo and phantom measurements were comparable.
Conclusions
Our simulation framework quantitatively predicts the different impact of B1+ mapping techniques on permittivity reconstruction and shows high sensitivity of permittivity reconstructions to sequence‐specific bias and noise perturbation in the B1+ map. These findings are supported by the experimental results.
Purpose
A purely experimental method for MRI‐based transfer function (TF) determination is presented. A TF characterizes the potential for radiofrequency heating of a linear implant by relating the ...incident tangential electric field to a scattered electric field at its tip. We utilize the previously introduced transfer matrix (TM) to determine transfer functions solely from the MR measurable quantities, that is, the B1+ and transceive phase distributions. This technique can extend the current practice of phantom‐based TF assessment with dedicated experimental setup toward MR‐based methods that have the potential to assess the TF in more realistic situations.
Theory and Methods
An analytical description of the B1+ magnitude and transceive phase distribution around a wire‐like implant was derived based on the TM. In this model, the background field is described using a superposition of spherical and cylindrical harmonics while the transfer matrix is parameterized using a previously introduced attenuated wave model. This analytical description can be used to estimate the transfer matrix and transfer function based on the measured B1+ distribution.
Results
The TF was successfully determined for 2 mock‐up implants: a 20‐cm bare copper wire and a 20‐cm insulated copper wire with 10 mm of insulation stripped at both endings in respectively 4 and 3 different trajectories. The measured TFs show a strong correlation with a reference determined from simulations and between the separate experiments with correlation coefficients above 0.96 between all TFs. Compared to the simulated TF, the maximum deviation in the estimated tip field is 9.4% and 12.2% for the bare and insulated wire, respectively.
Conclusions
A method has been developed to measure the TF of medical implants using MRI experiments. Jointly fitting the incident and scattered B1+ distributions with an analytical description based on the transfer matrix enables accurate determination of the TF of 2 test implants. The presented method no longer needs input from simulated data and can therefore, in principle, be used to measure TF's in test animals or corpses.
New magnetic resonance imaging (MRI) techniques that offer faster scanning and potential artificial intelligence-assisted interpretation and diagnosis can significantly impact existing workflows in ...radiology. In a qualitative study embedded within a responsible research and innovation design, we investigate the development and potential implementation of quantitative MRI. We aim to investigate postdigital MRI futures, covered by scenarios of potential workflows, as well as the resulting implications for professions and related education involved in the MRI process. Furthermore, we examine the related and changing responsibilities, more specifically reflecting on ‘forward-looking responsibilities’. Through expert interviews (
n
= 20) and a focus group, stakeholder perspectives on the future of quantitative imaging techniques were explored. During a subsequent co-creation workshop and another focus group, stakeholders reflected on future scenarios in quantitative MRI. Our study shows that a proactive and future-oriented investigation of the influence of emerging technologies on potential workflows and subsequent changes in expertise and roles help in gaining or increasing awareness about the wider impact of a technology developed to contribute to faster and quantitative MRI exams. We argue that anticipating postdigital worlds by reflecting on future responsibilities through the co-creation of imaginaries can help making uncertain futures tangible in other ways.
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