Evaluating the role of 1.5T quantitative susceptibility mapping for subthalamic nucleus targeting in deep brain stimulation surgery.

BACKGROUND AND PURPOSE: Quantitative susceptibility mapping (QSM) has been shown to be valuable in direct targeting for subthalamic nucleus (STN) DBS, given its higher quality of contrast between the STN border and adjacent anatomical structures. The objective is to demonstrate the feasibility of using 1.5T QSM for direct targeting in STN DBS planning. MATERIAL AND METHODS: Eleven patients underwent MRI acquisitions using a 1.5T scanner, including multi-echo gradient echo sequences for generating QSM images. 22 STN targets were planned with direct targeting method using QSM images by one stereotactic neurosurgeon and indirect targeting method using standard protocol by a second stereotactic neurosurgeon. The two physicians were blinded to each other's results. RESULTS: The mean coordinates for the STN using direct targeting relative to the mid-commissural point (MCP) was 11.41±2.43mm lateral, 2.48±0.53mm posterior and 4.45±0.95mm inferior. The mean coordinates for the STN using indirect targeting was 11.79±2.51mm lateral, 2.55±0.54mm posterior, and 4.84±1.03mm inferior. The mean (±SEM) radial error between the direct and indirect target was 0.67±0.14mm. In cases where DBS electrodes were implanted, the radial difference between the indirect and actual target (1.19±0.30mm) was statistically equivalent to the radial difference between the direct and actual target (1.0±0.27mm). CONCLUSIONS: Direct targeting of the STN for DBS implantation using 1.5T QSM was found to be statistically equivalent to standard protocol surgery planning. This may offer a simpler, more intuitive alternative for DBS surgery planning at centers with 1.5T MRIs.

Improving Safety of MRI in Patients with Deep Brain Stimulation Devices.

MRI is a valuable clinical and research tool for patients undergoing deep brain stimulation (DBS). However, risks associated with imaging DBS devices have led to stringent regulations, limiting the clinical and research utility of MRI in these patients. The main risks in patients with DBS devices undergoing MRI are heating at the electrode tips, induced currents, implantable pulse generator dysfunction, and mechanical forces. Phantom model studies indicate that electrode tip heating remains the most serious risk for modern DBS devices. The absence of adverse events in patients imaged under DBS vendor guidelines for MRI demonstrates the general safety of MRI for patients with DBS devices. Moreover, recent work indicates that-given adequate safety data-patients may be imaged outside these guidelines. At present, investigators are primarily focused on improving DBS device and MRI safety through the development of tools, including safety simulation models. Existing guidelines provide a standardized framework for performing safe MRI in patients with DBS devices. It also highlights the possibility of expanding MRI as a tool for research and clinical care in these patients going forward.

Use of Functional Magnetic Resonance Imaging to Assess How Motor Phenotypes of Parkinson's Disease Respond to Deep Brain Stimulation.

BACKGROUND: Deep brain stimulation (DBS) is a well-accepted treatment of Parkinson's disease (PD). Motor phenotypes include tremor-dominant (TD), akinesia-rigidity (AR), and postural instability gait disorder (PIGD). The mechanism of action in how DBS modulates motor symptom relief remains unknown. OBJECTIVE: Blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) was used to determine whether the functional activity varies in response to DBS depending on PD phenotypes. MATERIALS AND METHODS: Subjects underwent an fMRI scan with DBS cycling ON and OFF. The effects of DBS cycling on BOLD activation in each phenotype were documented through voxel-wise analysis. For each region of interest, ANOVAs were performed using T-values and covariate analyses were conducted. Further, a correlation analysis was performed comparing stimulation settings to T-values. Lastly, T-values of subjects with motor improvement were compared to those who worsened. RESULTS: As a group, BOLD activation with DBS-ON resulted in activation in the motor thalamus (p < 0.01) and globus pallidus externa (p < 0.01). AR patients had more activation in the supplementary motor area (SMA) compared to PIGD (p < 0.01) and TD cohorts (p < 0.01). Further, the AR cohort had more activation in primary motor cortex (MI) compared to the TD cohort (p = 0.02). Implanted nuclei (p = 0.01) and phenotype (p = <0.01) affected activity in MI and phenotype alone affected SMA activity (p = <0.01). A positive correlation was seen between thalamic activation and pulse-width (p = 0.03) and between caudate and total electrical energy delivered (p = 0.04). CONCLUSIONS: These data suggest that DBS modulates network activity differently based on patient motor phenotype. Improved understanding of these differences may further our knowledge about the mechanisms of DBS action on PD motor symptoms and to optimize treatment.

Functional MRI Safety and Artifacts during Deep Brain Stimulation: Experience in 102 Patients.

BackgroundWith growing numbers of patients receiving deep brain stimulation (DBS), radiologists are encountering these neuromodulation devices at an increasing rate. Current MRI safety guidelines, however, limit MRI access in these patients.PurposeTo describe an MRI (1.5 T and 3 T) experience and safety profile in a large cohort of participants with active DBS systems and characterize the hardware-related artifacts on images from functional MRI.Materials and MethodsIn this prospective study, study participants receiving active DBS underwent 1.5- or 3-T MRI (T1-weighted imaging and gradient-recalled echo [GRE]-echo-planar imaging [EPI]) between June 2017 and October 2018. Short- and long-term adverse events were tracked. The authors quantified DBS hardware-related artifacts on images from GRE-EPI (functional MRI) at the cranial coil wire and electrode contacts. Segmented artifacts were then transformed into standard space to define the brain areas affected by signal loss. Two-sample t tests were used to assess the difference in artifact size between 1.5- and 3-T MRI.ResultsA total of 102 participants (mean age ± standard deviation, 60 years ± 11; 65 men) were evaluated. No MRI-related short- and long-term adverse events or acute changes were observed. DBS artifacts were most prominent near the electrode contacts and over the frontoparietal cortical area where the redundancy of the extension wire is placed subcutaneously. The mean electrode contact artifact diameter was 9.3 mm ± 1.6, and 1.9% ± 0.8 of the brain was obscured by the coil artifact. The coil artifacts were larger at 3 T than at 1.5 T, obscuring 2.1% ± 0.7 and 1.4% ± 0.7 of intracranial volume, respectively (P < .001). The superficial frontoparietal cortex and deep structures neighboring the electrode contacts were most commonly obscured.ConclusionWith a priori local safety testing, patients receiving deep brain stimulation may safely undergo 1.5- and 3-T MRI. Deep brain stimulation hardware-related artifacts only affect a small proportion of the brain.© RSNA, 2019Online supplemental material is available for this article.See also the editorial by Martin in this issue.

Electrical properties tomography: Available contrast and reconstruction capabilities.

MR-based electrical properties tomography converts the MRI transmit/receive RF field measurements to tissue electrical property maps through dedicated reconstruction algorithms. Recent reports showed that despite limitations, electrical properties tomography holds promise for generating additional contrast for tumor detection and patient-specific modeling of tissue-RF field interactions. This review summarizes the available tissue electrical property contrasts and compares them with the capabilities of the most commonly used electrical properties tomography reconstruction method. Future directions and prospects of clinical translation are discussed.

On the (Non-)equivalency of monopolar and bipolar settings for deep brain stimulation fMRI studies of Parkinson's disease patients.

BACKGROUND: The majority of Parkinson's disease patients with deep brain stimulation (DBS) use a monopolar configuration, which presents challenges for EEG and MRI studies. The literature reports algorithms to convert monopolar to bipolar settings. PURPOSE/HYPOTHESIS: To assess brain responses of Parkinson's disease patients implanted with DBS during fMRI studies using their clinical and presumed equivalent settings using a published conversion recipe. STUDY TYPE: Prospective. SUBJECTS: Thirteen DBS patients. FIELD STRENGTH/SEQUENCE: 1.5T and 3T, fMRI using gradient echo-planar imaging. ASSESSMENT: Patients underwent 30/30sec ON/OFF DBS fMRI scans using monopolar and bipolar settings. To convert to a bipolar setting, the negative contact used for the monopolar configuration remained constant and the adjacent dorsal contact was rendered positive, while increasing the voltage by 30%. fMRI activation/deactivation maps and motor Unified Parkinson's Disease Rating Scale (UPDRS-III) scores were compared for patients in both configurations. STATISTICAL TESTS: T-tests were used to compare UPDRS scores and volumes of tissue activated (VTA) diameters in monopolar and bipolar configurations. RESULTS: The patterns of fMRI activation in the monopolar and bipolar configurations were generally different. The thalamus, pallidum, and visual cortices exhibited higher activation using the patient's clinical settings than the presumed equivalent settings. VTA diameters were lower (7 mm vs. 6.3 mm, P = 0.047) and UPDRS scores were generally higher in the bipolar (33.2 ± 16) than in the monopolar configuration (28.3 ± 17.4), without reaching statistical significance (P > 0.05). DATA CONCLUSION: Monopolar and bipolar configurations result in different patterns of brain activation while using a previously published monopolar-bipolar conversion algorithm. Clinical benefits may be achieved with varying patterns of brain responses. Blind conversion from one to the other should be avoided for purposes of understanding the mechanisms of DBS. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018.

Quantitative in vivo proton MR spectroscopic assessment of lipid metabolism: Value for breast cancer diagnosis and prognosis.

BACKGROUND: Breast magnetic resonance spectroscopy (1 H-MRS) has been largely based on choline metabolites; however, other relevant metabolites can be detected and monitored. PURPOSE: To investigate whether lipid metabolite concentrations detected with 1 H-MRS can be used for the noninvasive differentiation of benign and malignant breast tumors, differentiation among molecular breast cancer subtypes, and prediction of long-term survival outcomes. STUDY TYPE: Retrospective. SUBJECTS: In all, 168 women, aged ≥18 years. FIELD STRENGTH/SEQUENCE: Dynamic contrast-enhanced MRI at 1.5 T: sagittal 3D spoiled gradient recalled sequence with fat saturation, flip angle = 10°, repetition time / echo time (TR/TE) = 7.4/4.2 msec, slice thickness = 3.0 mm, field of view (FOV) = 20 cm, and matrix size = 256 × 192. 1 H-MRS: PRESS with TR/TE = 2000/135 msec, water suppression, and 128 scan averages, in addition to 16 reference scans without water suppression. ASSESSMENT: MRS quantitative analysis of lipid resonances using the LCModel was performed. Histopathology was the reference standard. STATISTICAL TESTS: Categorical data were described using absolute numbers and percentages. For metric data, means (plus 95% confidence interval [CI]) and standard deviations as well as median, minimum, and maximum were calculated. Due to skewed data, the latter were more adequate; unpaired Mann-Whitney U-tests were performed to compare groups without and with Bonferroni correction. ROC analyses were also performed. RESULTS: There were 111 malignant and 57 benign lesions. Mean voxel size was 4.4 ± 4.6 cm3 . Six lipid metabolite peaks were quantified: L09, L13 + L16, L21 + L23, L28, L41 + L43, and L52 + L53. Malignant lesions showed lower L09, L21 + L23, and L52 + L53 than benign lesions (P = 0.022, 0.027, and 0.0006). Similar results were observed for Luminal A or Luminal A/B vs. other molecular subtypes. At follow-up, patients were split into two groups based on median values for the six peaks; recurrence-free survival was significantly different between groups for L09, L21 + L23, and L28 (P = 0.0173, 0.0024, and 0.0045). DATA CONCLUSION: Quantitative in vivo 1 H-MRS assessment of lipid metabolism may provide an additional noninvasive imaging biomarker to guide therapeutic decisions in breast cancer. LEVEL OF EVIDENCE: 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:239-249.

EKG-based detection of deep brain stimulation in fMRI studies.

PURPOSE: To assess the impact of synchronization errors between the assumed functional MRI paradigm timing and the deep brain stimulation (DBS) on/off cycling using a custom electrocardiogram-based triggering system METHODS: A detector for measuring and predicting the on/off state of cycling deep brain stimulation was developed and tested in six patients in office visits. Three-electrode electrocardiogram measurements, amplified by a commercial bio-amplifier, were used as input for a custom electronics box (e-box). The e-box transformed the deep brain stimulation waveforms into transistor-transistor logic pulses, recorded their timing, and propagated it in time. The e-box was used to trigger task-based deep brain stimulation functional MRI scans in 5 additional subjects; the impact of timing accuracy on t-test values was investigated in a simulation study using the functional MRI data. RESULTS: Following locking to each patient's individual waveform, the e-box was shown to predict stimulation onset with an average absolute error of 112 ± 148 ms, 30 min after disconnecting from the patients. The subsecond accuracy of the e-box in predicting timing onset is more than adequate for our slow varying, 30-/30-s on/off stimulation paradigm. Conversely, the experimental deep brain stimulation onset prediction accuracy in the absence of the e-box, which could be off by as much as 4 to 6 s, could significantly decrease activation strength. CONCLUSIONS: Using this detector, stimulation can be accurately synchronized to functional MRI acquisitions, without adding any additional hardware in the MRI environment. Magn Reson Med 79:2432-2439, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Distortion correction in diffusion-weighted imaging of the breast: Performance assessment of prospective, retrospective, and combined (prospective + retrospective) approaches.

PURPOSE: To compare the effectiveness of prospective, retrospective, and combined (prospective + retrospective) EPI distortion correction methods in bilateral breast diffusion-weighted imaging (DWI) scans. METHODS: Five healthy female subjects underwent an axial bilateral breast DWI exam with and without prospective B0 inhomogeneity correction using slice-by-slice linear shimming. In each case, an additional b=0 DWI scan was performed with the polarity of the phase-encoding gradient reversed, to generate an estimated B0 map; this map or a separately acquired B0 map was used for retrospective correction, either alone or in combination with the prospective correction. The alignment between an undistorted, anatomical reference scan with similar contrast and the corrected b=0 DWI images with different correction schemes was assessed. RESULTS: The average cross-correlation coefficient between the DWI images and the anatomical reference scan was increased from 0.82 to 0.92 over the five volunteers when combined prospective and retrospective distortion correction was applied. Furthermore, such correction substantially reduced patient-to-patient variation of the image alignment and the variability of the average apparent diffusion coefficient in normal glandular tissue. CONCLUSION: Combined prospective and retrospective distortion correction can provide an efficient way to reduce susceptibility-induced image distortions and enhance the reliability of breast DWI exams. Magn Reson Med 78:247-253, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Flexible, 31-Channel breast coil for enhanced parallel imaging performance at 3T.

PURPOSE: To design, build, and characterize the performance of a novel 3T, 31-channel breast coil. METHODS: A flexible breast coil, accommodating all breast sizes while preserving close to unity filling factors in all configurations, was designed and built. Its performance was compared to the performance of the current state-of-the-art, 16 channel breast coil (Sentinelle coil, Hologic, Bedford, MA, USA), in phantoms and in vivo. RESULTS: Better axilla coverage and lower inter-coil coupling (12% versus 26%, as characterized by the average off-diagonal elements of the noise correlation matrix) was exhibited by our 31-channel coil compared with the 16-channel coil. Breast area signal-to-noise ratio increases of 68% (phantom) and 28% ± 31% (in vivo) were observed when the 31-channel coil was used. For the 31-channel/16-channel arrays, respectively, two-dimensional acceleration factors of left/right × superior/inferior = 4.3 × 2.4 resulted in average g-factors of 1.10/1.68 (in vitro) and 1.28/2.75 (in vivo); acceleration factors of left/right × anterior/posterior = 3.0 × 2.8 resulted in average g-factors of 1.06/1.54 (in vitro) and 1.05/1.12 (in vivo). CONCLUSION: A high performance breast coil was built; its capabilities were demonstrated in phantom and normal volunteer imaging experiments.

On conductivity, permittivity, apparent diffusion coefficient, and their usefulness as cancer markers at MRI frequencies.

PURPOSE: To investigate the permittivity and conductivity of cancerous and normal tissues, their correlation to the apparent diffusion coefficient (ADC), and the specificity that they could add to cancer detection. THEORY: Breast and prostate carcinomas were induced in rats. Conductivity and permittivity measurements were performed in the anesthetized animals using a dielectric probe and an impedance analyzer between 50 and 270 MHz. The correlations between ADCs (measured at 128 MHz) and conductivity values were investigated. Frequency-dependent discriminant functions were computed to assess the value that each parameter adds to cancer detection. METHODS: Tumors exhibited higher permittivity than muscle tissue by 27%/12%/5% at 64/128/270MHz. Frequency independent, 15-20% higher conductivity was also noted in tumors compared to muscle tissue over the same frequency range. Strong negative correlation was observed between tissue conductivity and ADC. Whereas permittivity had the strongest discriminatory power at 64 MHz, it became comparable to ADC at 128 MHz and less important than ADC at 270 MHz. CONCLUSION: Conductivity measurements offered limited advantages in separating cancer from normal tissue beyond what ADC already provided; conversely, permittivity added separation power when added to the discriminant function. The moderately high cancerous tissue permittivity and conductivity impose strong constraints on the capability of MRI-based tissue electrical property measurements.

Dynamic slice-dependent shim and center frequency update in 3 T breast diffusion weighted imaging.

PURPOSE: To demonstrate dynamic slice-dependent shim update as a simple method to reduce susceptibility-induced B0 inhomogeneity and associated pixel shift artifacts in diffusion-weighted echo planar imaging (DW-EPI) in 3 T breast imaging. METHODS: Dynamic slice-dependent update of linear shim and center frequency was implemented in a dual-echo B0 mapping sequence and a DW-EPI sequence. Multi-slice axial B0 maps and diffusion-weighted images were obtained from four volunteers with both conventional and dynamic shim methods. The two shim methods were compared in terms of B0 homogeneity and EPI pixel shift artifacts. RESULTS: In all volunteers the B0 maps showed significantly improved homogeneity; the left-right asymmetry was reduced by 79% and within-slice B0 standard deviation was reduced by 20% on the average. The improvements were better than what was previously reported for conventional (static) third-order shim in bilateral breast. Anatomy-referenced apparent diffusion coefficient (ADC) maps showed reduced overall image registration error obtainable with dynamic shim. CONCLUSIONS: Dynamic shim is an effective method to improve B0 shimming and DW-EPI image quality in 3 T bilateral breast imaging. Magn Reson Med 71:1813-1818, 2014. © 2013 Wiley Periodicals, Inc.

On shimming approaches in 3T breast MRI.

A comparative study is presented, analyzing quantitatively the impact of 15 shim strategies on the homogeneity of the main magnetic field over the three-dimensional breast region in 3T MRI. The results obtained in 12 female volunteers, spanning a wide range of body and breast types, indicate that the inclusion of the back and heart in the shim region of interest leads to considerable decrease in field homogeneity, and needs to be avoided. Comparison between shim strategies using volumetric B(0) maps, covering the entire breast region, and 1-6 plane B(0) maps indicate only minimally reduced performance for the latter. Interestingly, however, no single shim strategy relying on a limited number of B(0) maps as input was found to work best in all volunteers. This was attributed to the limited capability of a small number of B(0) maps to capture the B(0) variability existent within breast. On the average, a rectangular shim region of interest, encompassing the breast region alone, worked best for the cohort studied here.

Patient-to-patient variation of susceptibility-induced B0 field in bilateral breast MRI.

PURPOSE: To evaluate intersubject variability of susceptibility-induced static field inhomogeneity in breast and to assess effectiveness of whole-body high-order shimming applied to bilateral breast. MATERIALS AND METHODS: A fast, computationally efficient method to calculate susceptibility-induced static field from anatomical images was developed. The method was validated against the conventional multiecho B(0) mapping method and was used to generate data for linear and higher-order shim simulation on 13 volunteers. RESULTS: Most volunteers showed a significant anterior-posterior B(0) gradient. The majority of the subjects also exhibited a statistically significant left-right gradient. The second- and third-order shimming provided only minor (<5% each) improvement in B(0) homogeneity. CONCLUSION: The shape of the air-tissue boundary determines most of the observed B(0) distribution in bilateral breast. Despite significant variability among subjects, a common feature traceable to generic anatomy exists in the linear gradient. Nonlinear variation of susceptibility-induced B(0) field occurs over a relatively short length scale and is likely best shimmed by slice-dependent or localized shimming.

Field shaping arrays: a means to address shading in high field breast MRI.

PURPOSE: To develop a simple correction approach to mitigate shading in 3 Tesla (T) breast MRI. MATERIALS AND METHODS: A slightly modified breast receive (Rx) array, which we termed field shaping array (FSA), was shown to mitigate breast shading at 3T. In this FSA, one Rx element was selectively unblocked and tuned off the Larmor frequency during the transmit (Tx) phase. The current flowing in this element during Tx created a secondary transmit field; the vector addition of this field and the one created directly by the body coil resulted in a more uniform excitation profile over the entire breast area. The receive Rx element was returned to its intended tuning during the Rx phase, ensuring unperturbed signal reception. RESULTS: Using the FSA, improved Tx field uniformity, better fat suppression, increased image homogeneity and reduced power deposition was seen in all volunteers studied. CONCLUSION: A simple modification of a standard breast Rx array, converting it to a field shaping array, was shown to mitigate breast shading in all volunteers studied.

Improved myo-inositol detection through Carr-Purcell PRESS: a tool for more sensitive mild cognitive impairment diagnosis.

A 3-T study is presented, comparing the ability of two (1) H spectroscopy pulse sequences, Carr-Purcell point resolved spectroscopy (CPRESS; TE = 45 msec), and conventional PRESS (TE = 35 msec), to separate between groups of 20 normal control (NC) and 20 mild cognitive impairment (MCI) subjects. Both sequences showed higher myo-inositol (mI) and mI/N-acetyl-aspartate (NAA) levels in the posterior cingulate gyrus of the MCI subjects. The increased intrasubject repeatability of mI and mI/NAA CPRESS measurements (∼ 6% vs. 9% for PRESS) translated into decreased intraclass variability. A 22% intraclass mI PRESS variability was reduced to 16% for CPRESS, and an 18% intraclass mI/NAA PRESS variability was reduced to 12% for CPRESS for the group of NC subjects. Similar results were observed for the MCI subjects. Decreased intraclass variability led to improved separation between NC and MCI subjects (P = 0.017 for PRESS and P < 0.0001 for CPRESS mI/NAA, the best NC/MCI discriminant for each method). Seventy-five percent sensitivity at eighty percent specificity was demonstrated by mI/NAA CPRESS measurements in separating NC from MCI subjects. High correlations were also observed between subject performance on a number of neuropsychological tests (probing verbal memory, visuoconstruction performance, and visual motor integration) and the mI/NAA ratio; higher correlation coefficients (with stronger statistical significance) were consistently evident for CPRESS than for PRESS data.

Fast radiofrequency flip angle calibration by Bloch-Siegert shift.

In a recent work, we presented a novel method for B 1+ field mapping based on the Bloch-Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off-resonance radiofrequency pulses were added to a slice-selective spin echo sequence. The off-resonance pulses induce a Bloch-Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B(1) (2) . The signal is further spatially localized by a readout gradient, and the signal-weighted average B(1) field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch-Siegert-based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers.

The case of the missing glutamine.

A theoretical study was performed to determine the accuracy and repeatability of multiple one-dimensional pulse sequences in the quantification of glutamine concentration at 3 T. Variable repeatability (12% to > 50%) and significant absolute error (-50% to +70%) were noted for the eight pulse sequences considered. Data acquired in vivo using three of the pulse sequences used for simulation matched the predicted repeatability well; among the pulse sequences considered, point-resolved spectroscopy (TE = 80 ms) offered minimal error and acceptable repeatability (12%) for brain glutamine measurements. Following correction for the expected bias of each pulse sequence, consistent glutamine measurements, in the 1-mM range, were reported with the three sequences. An explanation for the mismatch between in vivo (1)H MRS and in vitro (13)C/(1)H MRS at high field was attempted.

Local specific absorption rate in high-pass birdcage and transverse electromagnetic body coils for multiple human body models in clinical landmark positions at 3T.

PURPOSE: To use electromagnetic (EM) simulations to study the effects of body type, landmark position, and radiofrequency (RF) body coil type on peak local specific absorption rate (SAR) in 3T magnetic resonance imaging (MRI). MATERIALS AND METHODS: Numerically computed peak local SAR for four human body models (HBMs) in three landmark positions (head, heart, pelvic) were compared for a high-pass birdcage and a transverse electromagnetic 3T body coil. Local SAR values were normalized to the IEC whole-body average SAR limit of 2.0 W/kg for normal scan mode. RESULTS: Local SAR distributions were highly variable. Consistent with previous reports, the peak local SAR values generally occurred in the neck-shoulder area, near rungs, or between tissues of greatly differing electrical properties. The HBM type significantly influenced the peak local SAR, with stockier HBMs, extending extremities towards rungs, displaying the highest SAR. There was also a trend for higher peak SAR in the head-centric and heart-centric positions. The impact of the coil types studied was not statistically significant. CONCLUSION: The large variability in peak local SAR indicates the need to include more than one HBM or landmark position when evaluating safety of body coils. It is recommended that an HBM with arms near the rungs be included to create physically realizable high-SAR scenarios.

Use of Functional MRI to Assess Effects of Deep Brain Stimulation Frequency Changes on Brain Activation in Parkinson Disease.

BACKGROUND: Models have been developed for predicting ideal contact and amplitude for subthalamic nucleus (STN) deep brain stimulation (DBS) for Parkinson disease (PD). Pulse-width is generally varied to modulate the size of the energy field produced. Effects of varying frequency in humans have not been systematically evaluated. OBJECTIVE: To examine how altered frequencies affect blood oxygen level-dependent activation in PD. METHODS: PD subjects with optimized DBS programming underwent functional magnetic resonance imaging (fMRI). Frequency was altered and fMRI scans/Unified Parkinson Disease Rating Scale motor subunit (UPDRS-III) scores were obtained. Analysis using DBS-OFF data was used to determine which regions were activated during DBS-ON. Peak activity utilizing T-values was obtained and compared. RESULTS: At clinically optimized settings (n = 14 subjects), thalamic, globus pallidum externa (GPe), and posterior cerebellum activation were present. Activation levels significantly decreased in the thalamus, anterior cerebellum, and the GPe when frequency was decreased (P < .001). Primary somatosensory cortex activation levels significantly decreased when frequency was increased by 30 Hz, but not 60 Hz. Sex, age, disease/DBS duration, and bilaterality did not significantly affect the data. Retrospective analysis of fMRI activation patterns predicted optimal frequency in 11/14 subjects. CONCLUSION: We show the first data with fMRI of STN DBS-ON while synchronizing cycling with magnetic resonance scanning. At clinically optimized settings, an fMRI signature of thalamic, GPe, and posterior cerebellum activation was seen. Reducing frequency significantly decreased thalamic, GPe, and anterior cerebellum activation. Current standard-of-care programming can take up to 6 mo using UPDRS-III testing alone. We provide preliminary evidence that using fMRI signature of frequency may have clinical utility and feasibility.