Apparent tissue sodium concentrations (aTSCs) determined by 23Na brain magnetic resonance imaging (MRI) have the potential to serve as a biomarker in pathologies such as multiple sclerosis (MS). ...However, the quantification is hindered by the intrinsically low signal‐to‐noise ratio of 23Na MRI. The purpose of this study was to improve the accuracy and reliability of quantitative 23Na brain MRI by implementing a dedicated postprocessing pipeline and to evaluate the applicability of the developed approach for the examination of MS patients. 23Na brain MRI measurements of 13 healthy volunteers and 17 patients with secondary progressive multiple sclerosis (SPMS) were performed at 7 T using a dual‐tuned 23Na/1H birdcage coil with a receive‐only 32‐channel phased array. The aTSC values were determined for normal appearing white matter (NAWM) and normal appearing gray matter (NAGM) in healthy subjects and SPMS patients. Signal intensities were normalized using the mean cerebrospinal fluid (CSF) sodium concentration determined in 37 separate patients receiving a spinal tap for routine diagnostic purposes. Five volunteers underwent MRI examinations three times in a row to assess repeatability. Coefficients of variation (CoVs) were used to quantify the repeatability of the proposed method. aTSC values were compared regarding brain regions and subject cohort using the paired‐samples Wilcoxon rank‐sum test. Laboratory CSF sodium concentration did not differ significantly between patients without and with MS (p = 0.42). The proposed quantification workflow for 23Na MRI was highly repeatable with CoVs averaged over all five volunteers of 1.9% ± 0.9% for NAWM and 2.2% ± 1.6% for NAGM. Average NAWM aTSC was significantly higher in patients with SPMS compared with the control group (p = 0.009). Average NAGM aTSC did not differ significantly between healthy volunteers and MS patients (p = 0.98). The proposed postprocessing pipeline shows high repeatability and the results can serve as a baseline for further studies establishing 23Na brain MRI as a biomarker in diseases such as MS.
The accuracy and reliability of quantitative 23Na brain MRI was improved by implementing a dedicated postprocessing pipeline. Sodium concentrations of brain tissues were determined in healthy subjects and patients with MS. Signal intensities were normalized using the mean cerebrospinal fluid sodium concentration determined in separate patients receiving a spinal tap. The proposed approach shows high repeatability, and the results can serve as a baseline for further studies establishing 23Na brain MRI as a biomarker in diseases such as MS.
Purpose
In this work, we investigated the ability of neural networks to rapidly and robustly predict Lorentzian parameters of multi‐pool CEST MRI spectra at 7 T with corresponding uncertainty maps to ...make them quickly and easily available for routine clinical use.
Methods
We developed a deepCEST 7 T approach that generates CEST contrasts from just 1 scan with robustness against B1 inhomogeneities. The input data for a neural feed‐forward network consisted of 7 T in vivo uncorrected Z‐spectra of a single B1 level, and a B1 map. The 7 T raw data were acquired using a 3D snapshot gradient echo multiple interleaved mode saturation CEST sequence. These inputs were mapped voxel‐wise to target data consisting of Lorentzian amplitudes generated conventionally by 5‐pool Lorentzian fitting of normalized, denoised, B0‐ and B1‐corrected Z‐spectra. The deepCEST network was trained with Gaussian negative log‐likelihood loss, providing an uncertainty quantification in addition to the Lorentzian amplitudes.
Results
The deepCEST 7 T network provides fast and accurate prediction of all Lorentzian parameters also when only a single B1 level is used. The prediction was highly accurate with respect to the Lorentzian fit amplitudes, and both healthy tissues and hyperintensities in tumor areas are predicted with a low uncertainty. In corrupted cases, high uncertainty indicated wrong predictions reliably.
Conclusion
The proposed deepCEST 7 T approach reduces scan time by 50% to now 6:42 min, but still delivers both B0‐ and B1‐corrected homogeneous CEST contrasts along with an uncertainty map, which can increase diagnostic confidence. Multiple accurate 7 T CEST contrasts are delivered within seconds.
Purpose
To enable whole‐brain quantitative CEST MRI at ultra‐high magnetic field strengths (B
0
≥ 7T) within short acquisition times.
Methods
Multiple interleaved mode saturation (MIMOSA) was ...combined with fast online‐customized (FOCUS) parallel transmission (pTx) excitation pulses and
correction to achieve homogenous whole‐brain coverage. Examinations of 13 volunteers were performed on a 7T MRI system with 3 different types of pulse sequences: (1) saturation in circular polarized (CP) mode and CP mode readout, (2) MIMOSA and CP readout, and (3) MIMOSA and FOCUS readout. For comparison, the inverse magnetic transfer ratio metric for relayed nuclear Overhauser effect and amide proton transfer were calculated. To investigate the number of required acquisitions for a good
correction, 4 volunteers were measured with 6 different B
1
amplitudes. Finally, time point repeatability was investigated for 6 volunteers.
Results
MIMOSA FOCUS sequence using
correction, with both single and multiple points, reduced inhomogeneity of the CEST contrasts around the occipital lobe and cerebellum. Results indicate that the most stable inter‐subject coefficient of variation was achieved using the MIMOSA FOCUS sequence. Time point repeatability of MIMOSA FOCUS with single‐point
correction showed a maximum coefficient of variation below 8% for 3 measurements in a single volunteer.
Conclusion
A combination of MIMOSA FOCUS with a single‐point
correction can be used to achieve quantitative CEST measurements at ultra‐high magnetic field strengths. Compared to previous
correction methods, acquisition time can be reduced as additional scans required for
correction can be omitted.
The objective of the current study was to optimize the postprocessing pipeline of 7 T chemical exchange saturation transfer (CEST) imaging for reproducibility and to prove this optimization for the ...detection of age differences and differences between patients with Parkinson's disease versus normal subjects. The following 7 T CEST MRI experiments were analyzed: repeated measurements of a healthy subject, subjects of two age cohorts (14 older, seven younger subjects), and measurements of 12 patients with Parkinson's disease. A slab‐selective,
B1+‐homogeneous parallel transmit protocol was used. The postprocessing, consisting of motion correction, smoothing,
B0‐correction, normalization, denoising,
B1+‐correction and Lorentzian fitting, was optimized regarding the intrasubject and intersubject coefficient of variation (CoV) of the amplitudes of the amide pool and the aliphatic relayed nuclear Overhauser effect (rNOE) pool within the brain. Seven “tricks” for postprocessing accomplished an improvement of the mean voxel CoV of the amide pool and the aliphatic rNOE pool amplitudes of less than 5% and 3%, respectively. These postprocessing steps are: motion correction with interpolation of the motion of low‐signal offsets (1) using the amide pool frequency offset image as reference (2), normalization of the Z‐spectrum using the outermost saturated measurements (3),
B0 correction of the Z‐spectrum with moderate spline smoothing (4), denoising using principal component analysis preserving the 11 highest intensity components (5),
B1+ correction using a linear fit (6) and Lorentzian fitting using the five‐pool fit model (7). With the optimized postprocessing pipeline, a significant age effect in the amide pool can be detected. Additionally, for the first time, an aliphatic rNOE contrast between subjects with Parkinson's disease and age‐matched healthy controls in the substantia nigra is detected. We propose an optimized postprocessing pipeline for CEST multipool evaluation. It is shown that by the use of these seven “tricks”, the reproducibility and, thus, the statistical power of a CEST measurement, can be greatly improved and subtle changes can be detected.
The presented postprocessing pipeline provides increased homogeneity and reproducibility. The mean voxel CoV is decreased to less than 5% for amide and to less than 3% for aliphatic rNOE contrast. Healthy aging‐related changes and altered aliphatic rNOE pools in the substantia nigra of patients in the early stages of Parkinson's disease are detected.
To demonstrate direct imaging of the white matter ultrashort T2∗ components at 7 Tesla using inversion recovery (IR)-enhanced ultrashort echo time (UTE) MRI. To investigate its characteristics, ...potentials and limitations, and to establish a clinical protocol.
The IR UTE technique suppresses long T2∗ signals within white matter by using adiabatic inversion in combination with dual-echo difference imaging. Artifacts arising at 7 T from long T2∗ scalp fat components were reduced by frequency shifting the IR pulse such that those frequencies were inverted likewise. For 8 healthy volunteers, the T2∗ relaxation times of white matter were then quantified. In 20 healthy volunteers, the UTE difference and fraction contrast were evaluated. Finally, in 6 patients with multiple sclerosis (MS), the performance of the technique was assessed.
A frequency shift of −1.2 ppm of the IR pulse (i.e. towards the fat frequency) provided a good suppression of artifacts. With this, an ultrashort compartment of (68 ± 6) % with a T2∗ time of (147 ± 58) μs was quantified with a chemical shift of (−3.6 ± 0.5) ppm from water. Within healthy volunteers' white matter, a stable ultrashort T2∗ fraction contrast was calculated. For the MS patients, a significant fraction reduction in the identified lesions as well as in the normal-appearing white matter was observed.
The quantification results indicate that the observed ultrashort components arise primarily from myelin tissue. Direct IR UTE imaging of the white matter ultrashort T2∗ components is thus feasible at 7 T with high quantitative inter-subject repeatability and good detection of signal loss in MS.
•Direct imaging of the white matter ultrashort T2∗ components at 7 Tesla is feasible.•Inversion recovery enhanced ultrashort echo time imaging provides robust contrast.•Artifacts from scalp lipid signals are resolved by shifting the inversion frequency.•The ultrashort T2∗ compartments in human white brain matter were quantified in vivo.•A clinically applicable protocol was demonstrated with high repeatability.
Isolated evaluation of multiparametric in vivo chemical exchange saturation transfer (CEST) MRI often requires complex computational processing for both correction of B0 and B1 inhomogeneity and ...contrast generation. For that, sufficiently densely sampled Z‐spectra need to be acquired. The list of acquired frequency offsets largely determines the total CEST acquisition time, while potentially representing redundant information. In this work, a linear projection‐based multiparametric CEST evaluation method is introduced that offers fast B0 and B1 inhomogeneity correction, contrast generation and feature selection for CEST data, enabling reduction of the overall measurement time. To that end, CEST data acquired at 7 T in six healthy subjects and in one brain tumor patient were conventionally evaluated by interpolation‐based inhomogeneity correction and Lorentzian curve fitting. Linear regression was used to obtain coefficient vectors that directly map uncorrected data to corrected Lorentzian target parameters. L1‐regularization was applied to find subsets of the originally acquired CEST measurements that still allow for such a linear projection mapping. The linear projection method allows fast and interpretable mapping from acquired raw data to contrast parameters of interest, generalizing from healthy subject training data to unseen healthy test data and to the tumor patient dataset. The L1‐regularization method shows that a fraction of the acquired CEST measurements is sufficient to preserve tissue contrasts, offering up to a 2.8‐fold reduction of scan time. Similar observations as for the 7‐T data can be made for data from a clinical 3‐T scanner. Being a fast and interpretable computation step, the proposed method is complementary to neural networks that have recently been employed for similar purposes. The scan time acceleration offered by the L1‐regularization (“CEST‐LASSO”) constitutes a step towards better applicability of multiparametric CEST protocols in a clinical context.
A data‐driven evaluation approach for multiparametric in vivo CEST MRI is proposed that allows mapping of uncorrected Z‐spectra to target contrasts (APT, NOE, MT, amine) by a simple, fast, and interpretable linear projection. Applying L1‐regularization–based feature selection (CEST‐LASSO) shows that a fraction of the originally acquired CEST measurements is sufficient to preserve tissue contrasts, offering up to a 2.8‐fold reduction of scan time. This represents a step towards better applicability of multiparametric CEST protocols in a clinical context.
Objectives
Metal artefacts can impair accurate diagnosis of haemorrhage using flat detector CT (FD-CT), especially after aneurysm coiling. Within this work we evaluate a prototype metal artefact ...reduction algorithm by comparison of the artefact-reduced and the non-artefact-reduced FD-CT images to pre-treatment FD-CT and multi-slice CT images.
Methods
Twenty-five patients with acute aneurysmal subarachnoid haemorrhage (SAH) were selected retrospectively. FD-CT and multi-slice CT before endovascular treatment as well as FD-CT data sets after treatment were available for all patients. The algorithm was applied to post-treatment FD-CT. The effect of the algorithm was evaluated utilizing the pre-post concordance of a modified Fisher score, a subjective image quality assessment, the range of the Hounsfield units within three ROIs, and the pre-post slice-wise Pearson correlation.
Results
The pre-post concordance of the modified Fisher score, the subjective image quality, and the pre-post correlation of the ranges of the Hounsfield units were significantly higher for artefact-reduced than for non-artefact-reduced images. Within the metal-affected slices, the pre-post slice-wise Pearson correlation coefficient was higher for artefact-reduced than for non-artefact-reduced images.
Conclusion
The overall diagnostic quality of the artefact-reduced images was improved and reached the level of the pre-interventional FD-CT images. The metal-unaffected parts of the image were not modified.
Key Points
•
After coiling subarachnoid haemorrhage, metal artefacts seriously reduce FD-CT image quality
.
•
This new metal artefact reduction algorithm is feasible for flat-detector CT
.
•
After coiling, MAR is necessary for diagnostic quality of affected slices
.
•
Slice-wise Pearson correlation is introduced to evaluate improvement of MAR in future studies
.
•
Metal-unaffected parts of image are not modified by this MAR algorithm
.
To demonstrate direct imaging of the white matter ultrashort T
components at 7 Tesla using inversion recovery (IR)-enhanced ultrashort echo time (UTE) MRI. To investigate its characteristics, ...potentials and limitations, and to establish a clinical protocol.
The IR UTE technique suppresses long T
signals within white matter by using adiabatic inversion in combination with dual-echo difference imaging. Artifacts arising at 7 T from long T
scalp fat components were reduced by frequency shifting the IR pulse such that those frequencies were inverted likewise. For 8 healthy volunteers, the T
relaxation times of white matter were then quantified. In 20 healthy volunteers, the UTE difference and fraction contrast were evaluated. Finally, in 6 patients with multiple sclerosis (MS), the performance of the technique was assessed.
A frequency shift of -1.2 ppm of the IR pulse (i.e. towards the fat frequency) provided a good suppression of artifacts. With this, an ultrashort compartment of (68 ± 6) % with a T
time of (147 ± 58) μs was quantified with a chemical shift of (-3.6 ± 0.5) ppm from water. Within healthy volunteers' white matter, a stable ultrashort T
fraction contrast was calculated. For the MS patients, a significant fraction reduction in the identified lesions as well as in the normal-appearing white matter was observed.
The quantification results indicate that the observed ultrashort components arise primarily from myelin tissue. Direct IR UTE imaging of the white matter ultrashort T
components is thus feasible at 7 T with high quantitative inter-subject repeatability and good detection of signal loss in MS.
Hintergrund: Die automatisierte Berechnung des Hirnvolumens aus MRT-3-D-Datensatzen erlaubt eine zeitsparende quantitative Beurteilung des Hirnvolumens in der klinischen Befundungsroutine. Ziel ...dieser Arbeit war es, zwei unabhangige Softwarelosungen hinsichtlich Sensitivitat und Validitat bei der Erkennung von volumengeminderten Hippocampussklerosen zu vergleichen. Methoden: Praoperative MRT-Bilder (1,5 T und 3 T, T1w MPRAGE sag 3D) von 25 Patienten mit histologisch gesicherter Hippocampussklerose (Typ 1: n = 20, Typ 2: n = 4, Typ 3: n = 1) wurden retrospektiv in die Untersuchung eingeschlossen. Es wurden die Softwareprogramme mdbrain Version 3.4.0 (Mediaire, Berlin) und Brain-Morphometry VB50B (Syngo.via, Siemens Healthineers, Erlangen) zur automatischen Volumenmessung des Hippocampus verwendet und mit der Klassifizierung durch 3 unabhangige Neuroradiologen anhand eines kompletten MRT-Epilepsieprotokolls verglichen. Ergebnisse: Die Sensitivitat fur die Erkennung einer Hippocampussklerose (anhand der Volumenminderung) lag bei 96,0% fur mdbrain und 32,0% fur BrainMorphometry. Die Spezifitat war 100% zum Goldstandard. Die berechneten Volumina unterschieden sich signifikant zwischen mdbrain und BrainMorphometry (3,2 ml vs. 2,7 ml; p < 0,005). Es bestand eine signifikante positive Korrelation (r = 0,79) zwischen den Volumina. Diskussion: KI-basierte Algorithmen sind geeignet Hippocampussklerosen anhand der Volumenminderung zu detektieren. Da es jedoch deutliche Unterschiede in den absoluten Volumenangaben und Bewertungen bezuglich pathologischer Werte gibt, mussen die Programme vor einer Routineanwendung auf ihre Eignung fur die jeweilige Fragestellung uberpruft werden. Die unterschiedlichen Sensitivitaten konnten u. a. durch unterschiedliche Grenzwerte fur den Volumennormbereich verursacht werden. Hippocampussklerosen ohne Volumenminderung konnen mit dieser Methode nicht detektiert werden. Fazit: Abhangig von der gewahlten Softwarelosung ist eine Hippocampussklerose (Volumenminderung) mit einer Sensitivitat von bis zu 92,3% bei einer Spezifitat von 100% detektierbar. So konnen KI-basierte Algorithmen den Radiologen unterstutzen und die bildgebende Epilepsiediagnostik verbessern bzw. beschleunigen.
Highlights • High quality MRI is very important in the diagnostic work-up of epilepsy. • VNS is increasingly used in the treatment of pharmacoresistant epilepsy. • MRI can be performed in ...VNS-patients under certain conditions. • Because of the resulting loss of image quality, subtle pathologies may be missed. • Image quality is adequate for detection and follow-up of ganglioglioma or PNH.
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