Low-Field-Strength Body MRI: Challenges and Opportunities at 0.55 T.

Advances in MRI technology have led to the development of low-field-strength (hereafter, "low-field") (0.55 T) MRI systems with lower weight, fewer shielding requirements, and lower cost than those of traditional (1.5-3 T) systems. The trade-offs of lower signal-to-noise ratio (SNR) at 0.55 T are partially offset by patient safety and potential comfort advantages (eg, lower specific absorption rate and a more cost-effective larger bore diameter) and physical advantages (eg, decreased T2* decay, shorter T1 relaxation times). Image reconstruction advances leveraging developing technologies (such as deep learning-based denoising) can be paired with traditional techniques (such as increasing the number of signal averages) to improve SNR. The overall image quality produced by low-field MRI systems, although perhaps somewhat inferior to 1.5-3 T MRI systems in terms of SNR, is nevertheless diagnostic for a broad variety of body imaging applications. Effective low-field body MRI requires (a) an understanding of the trade-offs resulting from lower field strengths, (b) an approach to modifying routine sequences to overcome SNR challenges, and (c) a workflow for carefully selecting appropriate patients. The authors describe the rationale, opportunities, and challenges of low-field body MRI; discuss important considerations for low-field imaging with common body MRI sequences; and delineate a variety of use cases for low-field body MRI. The authors also include lessons learned from their preliminary experience with a new low-field MRI system at a tertiary care center. Finally, they explore the future of low-field MRI, summarizing current limitations and potential future developments that may enhance the clinical adoption of this technology. ©RSNA, 2023 Supplemental material is available for this article. Quiz questions for this article are available through the Online Learning Center. See the invited commentary by Venkatesh in this issue.

An advanced self-care delivery model for leg ulcer management: a service evaluation.

BACKGROUND: Lower limb ulceration is a common cause of suffering in patients and its management poses a significant burden on the NHS, with venous leg ulcers (VLUs) being the most common hard-to-heal wound in the UK. It is estimated that over one million patients in the UK have lower limb ulceration, of which 560,000 were categorised as VLUs, with a cost burden of over £3 billion each year. OBJECTIVE: The aim of this service evaluation was to assess the effects of implementing a self-care delivery model on clinical outcomes with the intention of limiting face-to-face health professional contact to one appointment every 6 weeks. METHOD: A suitability assessment was conducted and a cohort of patients were moved to a self-care delivery model. Patient data were collected, anonymised and independently analysed, comparing time to healing against data on file from a previous report. RESULTS: This highlighted that, in 84 of the 95 patients selected, the VLUs had healed by week 24 on the pathway, a further 10 patients' VLUs had healed by week 42 and only one remaining patient reached 42 weeks without healing. CONCLUSION: These results support the hypothesis that patients with VLUs can self-care and deliver clinical effectiveness. It is recommended that all services explore the possibility of introducing a self-care model for VLU care.

Multisite evaluations of a T2 -relaxation-under-spin-tagging (TRUST) MRI technique to measure brain oxygenation.

PURPOSE: Venous oxygenation (Yv ) is an important index of brain physiology and may be indicative of brain diseases. A T2 -relaxation-under-spin-tagging (TRUST) MRI technique was recently developed to measure Yv . A multisite evaluation of this technique would be an important step toward broader availability and potential clinical utilizations of Yv measures. METHODS: TRUST MRI was performed on a total of 250 healthy subjects, 125 from the developer's site and 25 each from five other sites. All sites were equipped with a 3 Tesla (T) MRI of the same vendor. The estimated Yv and the standard error (SE) of the estimation εYv were compared across sites. RESULTS: The averaged Yv and εYv across six sites were 61.1% ± 1.4% and 1.3% ± 0.2%, respectively. Multivariate regression analysis showed that the estimated Yv was dependent on age (P = 0.009) but not on performance site. In contrast, the SE of the Yv estimation was site-dependent (P = 0.024) but was less than 1.5%. Further analysis revealed that εYv was positively associated with the amount of subject motion (P < 0.001) but negatively associated with blood signal intensity (P < 0.001). CONCLUSION: This work suggests that TRUST MRI can yield equivalent results of Yv estimation across different sites.

Postconcussion symptoms are associated with cerebral cortical thickness in healthy collegiate and preparatory school ice hockey players.

OBJECTIVE: To investigate the degree to which concussion history and postconcussive symptoms are associated with cortical morphology among male hockey players. STUDY DESIGN: Male subjects (n = 29), ranging in age from 14 to 23 years (mean 17.8 years), were recruited from preparatory school and collegiate ice hockey teams and underwent neuroimaging and baseline Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) testing. Cerebral cortical thickness was regressed against ImPACT Total Symptom Score (TSS), concussion history, as well as baseline measures of psychopathology. Reconstruction of surfaces and cortical thickness analysis were conducted with FreeSurfer (version 5.3.0). RESULTS: ImPACT TSS was inversely associated with local cortical thickness in widespread brain areas. Associations were revealed in a host of frontal as well as bilateral temporoparietal cortices. Conversely, concussion history was not associated with cortical thickness. An "Age by Concussion History" interaction was associated with thickness in the right ventrolateral and right parietal cortices. Post-hoc analysis revealed that concussed participants did not exhibit age-related cortical thinning in these regions. CONCLUSION: We have identified an association between brain structure and postconcussive symptoms among young, otherwise-healthy male athletes. Postconcussive symptoms and related reductions in cortical thickness may be tied to participation in a full-contact sport that involves frequent blows to the head.

In vivo quantitative whole-brain T1 rho MRI of multiple sclerosis.

BACKGROUND: To apply quantitative whole-brain T1 -rho (T1ρ ) and T2 imaging to the detection and quantification of brain changes resulting from multiple sclerosis (MS). METHODS: Twenty-three MS patients with clinically isolated syndrome (10) and relapsing remitting MS (13) phenotypes, compared with 24 age-matched healthy controls were imaged at 3 Tesla. An axial T1ρ -weighted three-dimensional turbo spin echo sequence with a variable flip angle and fluid suppression was used. Spin-lock times of 0, 20, 40, 60, 80, and 100 ms were used. Corresponding T2 maps were also acquired. RESULTS: Whole brain white matter (WM) T1ρ maps were elevated compared with controls (P = 0.002). WM lesion T1ρ and T2 values were highly correlated (r = 0.83), but T1ρ demonstrated 25% better contrast to noise ratio (P < 0.001). WM lesion T1ρ correlated with disease duration. Gray matter T1ρ was negatively correlated with the Expanded Disability Status Scale, r = -0.45, P = 0.03. Normal appearing gray matter and cortical gray matter lesions were negatively correlated on T1ρ , but not on T2 (rT1ρ = -0.63, pT1ρ = 0.03; rT2 = -0.17, pT2 = 0.6). CONCLUSION: T1ρ MRI demonstrates enhanced lesion contrast compared with T2 , and in some cases may provide complementary information. T1ρ may provide a useful measure of demyelinating processes in MS.

In vivo whole-brain T1-rho mapping across adulthood: normative values and age dependence.

PURPOSE: To report a systematic investigation of variations in brain T1ρ (T1-rho) values over adulthood, and present normative values for cortical gray matter, juxtacortical white matter, selected white matter tracts and subcortical structures. MATERIALS AND METHODS: Forty-one healthy control subjects (23 males aged 18-76 years and 18 females aged 21-73 years) were imaged at 3.0 Tesla (T) using a novel whole-brain T1ρ-weighted fluid suppressed three-dimensional turbo spin echo technique. T1ρ maps were calculated and atlas-based segmentation used to determine regional T1ρ values. Linear regression was used to determine changes in T1ρ with age. RESULTS: T1ρ values showed significant decreases with age in cortical gray matter (P < 0.001), left and right caudate (P = 0.004/0.006), putamen (P = 0.001, P < 0.001), hippocampus (P = 0.039/0.010), amygdala (P = 0.002/0.001), and nucleus accumbens (P = 0.001/0.002). Significant increases with age (P < 0.05) were observed in all the white matter tracts considered, except the corticospinal tracts and forceps major. CONCLUSION: T1ρ demonstrates changes related to the processes of normal aging.

In vivo quantification of T1ρ in lumbar spine disk spaces at 3 T using parallel transmission MRI.

OBJECTIVE: T1ρ MRI is an emerging, quantitative imaging modality that has been shown to correlate with proteoglycan content of disk material in vitro at 1.5 T. The purpose of this study is to quantify T1ρ values at all lumbar spine disk space levels at 3 T with parallel-transmission MRI in healthy adult volunteers. SUBJECTS AND METHODS: Thirty-four subjects (15 men, ages 21-60 years [mean age, 38.4 years]; and 19 women, ages 20-56 years [mean age, 36.5 years]) with no history of back pain or surgery underwent T1ρ MRI of the lumbar spine at 3 T with parallel transmission and sagittal T2-weighted imaging. Mean T1ρ values of all lumbar spine disk space levels were quantified. Linear regression analysis and Spearman rank correlation were performed on age, sex, degenerative grade (Pfirrmann scores), and T1ρ with significance set at p < 0.05 and correlations considered strong for r > 0.7 and moderate for r = 0.5-0.7. RESULTS: There was a statistically significant moderate negative correlation between T1ρ and subject age at disk space levels L1-2 through L4-5 (inclusive) (p < 0.001) and L5-S1 (p < 0.01). There was a statistically significant difference in T1ρ between all age groups sampled (p < 0.01) and a significant difference between T1ρ and Pfirrmann grades 1-3 (p < 0.01). CONCLUSION: T1ρ MRI in the lumbar spine with parallel transmission shows signifi-cant negative correlations with age at all disk space levels, which lends support to a potential role for T1ρ as a quantitative, in vivo biomarker of disk degeneration.

Cardiac reactive oxygen species after traumatic brain injury.

BACKGROUND: Cardiovascular complications after traumatic brain injury (TBI) contribute to morbidity and mortality and may provide a target for therapy. We examined blood pressure and left ventricle contractility after TBI, and tested the hypothesis that ß-adrenergic blockade would decrease oxidative stress after TBI. MATERIAL AND METHODS: Rodents received fluid-percussion injury or sham surgery, confirmed with magnetic resonance imaging (MRI) and histopathology. We followed recovery with sensorimotor coordination testing and blood pressure measurements. We assessed left ventricular ejection fraction using ECG-gated cardiac MRI and measured myocardial reactive oxygen species (ROS) with dihydroethidium. We randomized additional TBI and sham animals to postoperative treatment with propranolol or control, for measurement of ROS. RESULTS: Blood pressure and cardiac contractility were elevated 48 h after TBI. Myocardial tissue sections showed increased ROS. Treatment with propranolol diminished ROS levels following TBI. CONCLUSIONS: TBI is associated with increased cardiac contractility and myocardial ROS; decreased myocardial ROS after ß-blockade suggests that sympathetic stimulation is a mechanism of oxidative stress.

Improvements in lumbar spine MRI at 3 T using parallel transmission.

OBJECTIVE: Parallel transmission MRI at 3 T improves image quality by reducing dielectric effects with radiofrequency shimming. The purpose of this study was to determine whether parallel transmission MRI improves signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in lumbar spine MRI at 3 T. MATERIALS AND METHODS: Ten healthy volunteers underwent T1-weighted MRI and nine healthy volunteers underwent T2-weighted MRI of the lumbar spine. Sagittal and axial T1- and T2-weighted images were acquired using parallel transmission MRI and conventional MRI. The percentage improvements in SNR and CNR were calculated, and statistical significance was determined using a two-tailed Student t test with p < 0.05 for significance. RESULTS: The CNR and SNR showed statistically significant improvements at all levels of the lumbar spine except SNR at T11 on axial T2-weighted imaging. For sagittal T1-weighted imaging, the average improvement with parallel transmission MRI was 53% in CNR and 19% in SNR. For axial T1-weighted imaging, the average improvement was 48% in CNR and 23% in SNR. For sagittal T2-weighted imaging, the average CNR improvement was 38% and the average SNR improvement, 20%. For axial T2-weighted scans, the average percentage improvement in CNR was greater than 100% and the average SNR improvement was 18% with parallel transmission MRI. CONCLUSION: The parallel transmission sequence improves image quality of lumbar spine MRI at 3 T, which is quantitatively supported by statistically significant improvements in SNR and CNR.

Ultrafast 3-T MRI in the evaluation of children with acute lower abdominal pain for the detection of appendicitis.

OBJECTIVE: The purpose of this study is to evaluate the feasibility of ultrafast 3-T MRI in the evaluation of children with acute lower abdominal pain for the detection of appendicitis. SUBJECTS AND METHODS: Forty-two pediatric patients (30 girls and 12 boys; mean age, 11.5 years; age range, 4-17 years) with acute abdominal pain were prospectively studied. Ultrafast 3-T MRI was performed with a three-plane single-shot turbo spin-echo sequence and an axial T2-weighted turbo spin-echo sequence with fat suppression. All scans were performed without sedation or oral or IV contrast agent. Scan times were less than 8 minutes 45 seconds (median, 5 minutes 40 seconds). Patients underwent CT or ultrasound or both as a comparison study to the MRI examination. The MRI, CT, and ultrasound examinations were interpreted independently by four board-certified radiologists who were blinded to patient information, study interpretations, surgical pathologic findings, and final diagnosis. RESULTS: Twelve of 42 cases of acute appendicitis were detected with 100% sensitivity, 99% specificity, 100% negative predictive value, and 98% positive predictive value, all of which were statistically significant (p < 0.01). The pooled and individual receiver operating characteristic curves for radiologists' interpretation of the diagnosis of acute appendicitis were greater than 0.95 in all cases (p < 0.01) CONCLUSION: Ultrafast 3-T MRI is a feasible alternative imaging modality for the diagnosis of acute appendicitis in children, particularly in cases where ultrasound is equivocal or nondiagnostic, as an alternative to CT. Ultrafast MRI requires no sedation and no oral or IV contrast agent and has no associated radiation exposure risks.

Diffusion tensor imaging of the pediatric optic nerve: intrinsic and extrinsic pathology compared to normal controls.

PURPOSE: To establish normative magnetic resonance diffusion tensor imaging (MRDTI) data in the pediatric optic nerve and compare to pathologic conditions both intrinsic and extrinsic to the anterior optic nerve pathway. MATERIALS AND METHODS: A retrospective analysis of MRDTI at 3.0T in children ages 0-18 with both normal imaging studies and with pathologic conditions either arising from the optic nerves or exerting mass effect on the anterior optic pathways was performed. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were obtained within the posterior part of the intraorbital optic nerves. Statistical analysis compared normal controls to optic pathway lesions. RESULTS: Lesions intrinsic to the optic nerve including septo-optic dysplasia and optic nerve glioma demonstrated statistically significant reductions in FA and increases in ADC values. There was no statistically significant difference in FA or mean diffusivity between the normal controls and patients with lesions extrinsic to the optic nerve but exerting mass effect on the visual pathway. CONCLUSION: MRDTI is a feasible technique for evaluating the optic nerves in pediatric patients. It may play a role in presurgical planning by demonstrating integrity of the visual pathway in patients with resectable lesions, allowing for minimization of morbidity associated with vision loss.

Magnetic resonance diffusion tensor imaging and tractography of the lower spinal cord: application to diastematomyelia and tethered cord.

OBJECTIVES: To investigate the feasibility of routine clinical DTI of the lower spinal cord using high-field-strength MRI and parallel imaging, and to evaluate the utility of diffusion tensor imaging and tractography as tools for study of lower cord pathology. METHODS: Three patients with diastematomyelia, one patient with tethered cord, and six normal volunteers underwent MR imaging of the lower spine at 3 T. A 15-channel spine coil and parallel imaging were used with a six-direction single-shot echo-planar gradient echo technique. RESULTS: In normal volunteers, tractography delineated the conus and cauda equina. Tractography software permitted assessment of fractional anisotropy of the distal cord and nerve roots. In cases of tethered cord, tractography correlated with anatomical imaging. Tractography also correlated with the anatomical pathological findings in cases of diastematomyelia. CONCLUSIONS: The methods described enable routine DTI and tractography of the lower spinal cord at 3 T. Compared with conventional imaging, tractography offers additional information that may prove useful in the characterization and surgical planning for congenital lesions involving the lower spinal cord.

Impact of 3-Dimensional Versus 2-Dimensional Image Distortion Correction on Stereotactic Neurosurgical Navigation Image Fusion Reliability for Images Acquired With Intraoperative Magnetic Resonance Imaging.

BACKGROUND: Fusion of preoperative and intraoperative magnetic resonance imaging (iMRI) studies during stereotactic navigation may be very useful for procedures such as tumor resections but can be subject to error because of image distortion. OBJECTIVE: To assess the impact of 3-dimensional (3D) vs 2-dimensional (2D) image distortion correction on the accuracy of auto-merge image fusion for stereotactic neurosurgical images acquired with iMRI using a head phantom in different surgical positions. METHODS: T1-weighted intraoperative images of the head phantom were obtained using 1.5T iMRI. Images were postprocessed with 2D and 3D image distortion correction. These studies were fused to T1-weighted preoperative MRI studies performed on a 1.5T diagnostic MRI. The reliability of the auto-merge fusion of these images for 2D and 3D correction techniques was assessed both manually using the stereotactic navigation system and via image analysis software. RESULTS: Eight surgical positions of the head phantom were imaged with iMRI. Greater image distortion occurred with increased distance from isocenter in all 3 axes, reducing accuracy of image fusion to preoperative images. Visually reliable image fusions were accomplished in 2/8 surgical positions using 2D distortion correction and 5/8 using 3D correction. Three-dimensional correction yielded superior image registration quality as defined by higher maximum mutual information values, with improvements ranging between 2.3% and 14.3% over 2D correction. CONCLUSION: Using 3D distortion correction enhanced the reliability of surgical navigation auto-merge fusion of phantom images acquired with iMRI across a wider range of head positions and may improve the accuracy of stereotactic navigation using iMRI images.

Magnetic resonance diffusion tensor imaging of the optic nerves to guide treatment of pediatric suprasellar tumors.

BACKGROUND/AIMS: As a preoperative planning tool, conventional magnetic resonance (MR) imaging may have limited value in differentiating tumors from white matter tracts. MR diffusion tensor imaging (MRDTI) has become a useful tool for evaluating white matter tracts in relation to surrounding structures and has been used in surgical planning for brain tumors involving white matter. We investigated the use of DTI of the optic nerves in surgical planning for pediatric suprasellar tumors. METHODS: We present findings in 10 pediatric control patients and 2 cases of pediatric suprasellar tumors in which a routine 6-direction DTI of the brain was performed at 3 T. Postprocessing permitted the study of the diffusion tensor parameters, as well as the tractography, of the optic nerves. RESULTS: The control patients demonstrated the optimization of the technique and permitted the quantitation of fractional anisotropy and apparent diffusion coefficient values. The tumor cases demonstrated the utility of optic nerve DTI to differentiate between optic nerves and suprasellar/chiasmatic brain tumors. CONCLUSIONS: A routine 6-direction DTI of the pediatric brain at 3 T permits a detailed DTI study of the optic nerves. Optic nerve tractography can be used to aid in the evaluation and treatment of pediatric brain tumors in the area of the optic chiasm.

Dynamic changes in diffusion measures improve sensitivity in identifying patients with mild traumatic brain injury.

The goal of this study was to investigate patterns of axonal injury in the first week after mild traumatic brain injury (mTBI). We performed a prospective cohort study of 20 patients presenting to the emergency department with mTBI, using 3.0T diffusion tensor MRI immediately after injury and again at 1 week post-injury. Corresponding data were acquired from 16 controls over a similar time interval. Fractional anisotropy (FA) and other diffusion measures were calculated from 11 a priori selected axon tracts at each time-point, and the change across time in each region was quantified for each subject. Clinical outcomes were determined by standardized neurocognitive assessment. We found that mTBI subjects were significantly more likely to have changes in FA in those 11 regions of interest across the one week time period, compared to control subjects whose FA measurements were stable across time. Longitudinal imaging was more sensitive to these subtle changes in white matter integrity than cross-sectional assessments at either of two time points, alone. Analyzing the sources of variance in our control population, we show that this increased sensitivity is likely due to the smaller within-subject variability obtained by longitudinal analysis with each subject as their own control. This is in contrast to the larger between-subject variability obtained by cross-sectional analysis of each individual subject to normalized data from a control group. We also demonstrated that inclusion of all a priori ROIs in an analytic model as opposed to measuring individual ROIs improves detection of white matter changes by overcoming issues of injury heterogeneity. Finally, we employed genetic programming (a bio-inspired computational method for model estimation) to demonstrate that longitudinal changes in FA have utility in predicting the symptomatology of patients with mTBI. We conclude concussive brain injury caused acute, measurable changes in the FA of white matter tracts consistent with evolving axonal injury and/or edema, which may contribute to post-concussive symptoms.

Neuroimaging Biomarkers of a History of Concussion Observed in Asymptomatic Young Athletes.

Participation in contact sports places athletes at elevated risk for repeated head injuries and is associated with negative mental health outcomes later in life. The current study identified changes observable on neuroimaging that persisted beyond the apparent resolution of acute symptoms of concussion. Sixteen young adult ice hockey players with a remote history of concussion but no subjective complaints were compared against 13 of their teammates with no history of concussion. Participants completed a detailed phenotypic assessment and a neuroimaging battery including diffusion kurtosis imaging and resting-state functional magnetic resonance imaging. Athletes with a history of concussion performed no differently from those without on phenotypic assessment, but showed significantly elevated fractional anisotropy (FA) in the left genu and anterior corona radiata relative to those without. Post hoc analyses revealed that elevated FA was associated with increased microstructural complexity perpendicular to the primary axon (radial kurtosis). Athletes with concussion history also showed significant differences in the organization of the default mode network (DMN) characterized by stronger temporal coherence in posterior DMN, decreased temporal coherence in anterior DMN, and increased functional connectivity outside the DMN. In the absence of deficits on detailed phenotypic assessment, athletes with a history of concussion displayed changes to the microstructural architecture of the cerebral white matter and to the functional connectivity of the brain at rest. Some of these changes are consistent with those previously associated with persisting deficits and complaints, but we also report novel, complementary changes that possibly represent compensatory mechanisms.

Myelination in children with partial deletions of chromosome 18q.

BACKGROUND AND PURPOSE: We compared myelin levels in white matter (WM) in typically developing children with those of children with partial deletions of chromosome 18q (18q-). METHODS: Conventional spin-echo MR imaging at 1.9T was used to acquire T1-, T2-, and proton density-weighted images of the brain. From these images, a three-pool model was used to estimate the fraction of water in myelin, myelinated axon, and mixed water compartments (or pools) in six WM regions. A model curve was fit to the pool fractions to model the trend of myelin development by age in each region. Water-pool fractions in children with 18q- aged 5 months to 13 years were compared with those of age-matched, typically developing children. RESULTS: In children with 18q-, the model predicted later onset of myelination (P <.02), lower myelination rates (P <.001), and smaller equilibrium myelin pool fractions (P <.001). Significant differences were seen between the two groups for all three water pool fractions (P <.001). The mixed pool fraction was larger in children with 18q-. Although the myelin pool fraction was significantly smaller, the myelinated axon pool fraction was only slightly smaller, leading to a significantly smaller estimate of myelin per myelinated axon in children with 18q- (P <.001). CONCLUSION: Myelination modeling in 18q- children indicated delayed onset, a lower rate of myelination, and equilibrium myelin levels less than 50% those of age-matched, typically developing children.