Christopher Glielmi

DUAL PATHWAYS FOR HAPTIC AND VISUAL PERCEPTION OF SPATIAL AND TEXTURE INFORMATION

Segregation of information flow along a dorsally directed pathway for processing object location and a ventrally directed pathway for processing object identity is well established in the visual and auditory systems, but is less clear in the somatosensory system. We hypothesized that segregation of location vs. identity information in touch would be evident if texture is the relevant property for stimulus identity, given the salience of texture for touch. Here, we used functional magnetic resonance imaging (fMRI) to investigate whether the pathways for haptic and visual processing of location and texture are segregated, and the extent of bisensory convergence. Haptic texture-selectivity was found in the parietal operculum and posterior visual cortex bilaterally, and in parts of left inferior frontal cortex. There was bisensory texture-selectivity at some of these sites in posterior visual and left inferior frontal cortex. Connectivity analyses demonstrated, in each modality, flow of information from unisensory non-selective areas to modality-specific texture-selective areas and further to bisensory texture-selective areas. Location-selectivity was mostly bisensory, occurring in dorsal areas, including the frontal eye fields and multiple regions around the intraparietal sulcus bilaterally. Many of these regions received input from unisensory areas in both modalities. Together with earlier studies, the activation and connectivity analyses of the present study establish that somatosensory processing flows into segregated pathways for location and object identity information. The location-selective somatosensory pathway converges with its visual counterpart in dorsal frontoparietal cortex, while the texture-selective somatosensory pathway runs through the parietal operculum before converging with its visual counterpart in visual and frontal cortex. Both segregation of sensory processing according to object property and multisensory convergence appear to be universal organizing principles.

Correlation Between Standardized Uptake Value and Apparent Diffusion Coefficient of Neoplastic Lesions Evaluated With Whole-Body Simultaneous Hybrid PET/MRI

OBJECTIVE The purpose of this study was to assess the correlation between standardized uptake value (SUV) and apparent diffusion coefficient (ADC) of neoplastic lesions in the use of a simultaneous PET/MRI hybrid system. SUBJECTS AND METHODS Twenty-four patients with known primary malignancies underwent FDG PET/CT. They then underwent whole-body PET/MRI. Diffusion-weighted imaging was performed with free breathing and a single-shot spin-echo echo-planar imaging sequence with b values of 0, 350, and 750 s/mm(2). Regions of interest were manually drawn along the contours of neoplastic lesions larger than 1 cm, which were clearly identified on PET and diffusion-weighted images. Maximum SUV (SUVmax) on PET/MRI and PET/CT images, mean SUV (SUVmean), minimum ADC (ADCmin), and mean ADC (ADCmean) were recorded on PET/MR images for each FDG-avid neoplastic soft-tissue lesion with a maximum of three lesions per patient. Pearson correlation coefficient was used to asses the following relations: SUVmax versus ADCmin on PET/MR and PET/CT images, SUVmean versus ADCmean, and ratio of SUVmax to mean liver SUV (SUV ratio) versus ADCmin. A subanalysis of patients with progressive disease versus partial treatment response was performed with the ratio of SUVmax to ADCmin for the most metabolically active lesion. RESULTS Sixty-nine neoplastic lesions (52 nonosseous lesions, 17 bone metastatic lesions) were evaluated. The mean SUVmax from PET/MRI was 7.0 ± 6.0; SUVmean, 5.6 ± 4.6; mean ADCmin, 1.10 ± 0.58; and mean ADCmean, 1.48 ± 0.72. A significant inverse Pearson correlation coefficient was found between PET/MRI SUVmax and ADCmin (r = -0.21, p = 0.04), between SUVmean and ADCmean (r = -0.18, p = 0.07), and between SUV ratio and ADCmin (r = -0.27, p = 0.01). A similar inverse Pearson correlation coefficient was found between the PET/CT SUVmax and ADCmin. Twenty of 24 patients had previously undergone PET/CT; five patients had a partial treatment response, and six had progressive disease according to Response Evaluation Criteria in Solid Tumors 1.1. The ratio between SUVmax and ADCmin was higher among patients with progressive disease than those with a partial treatment response. CONCLUSION Simultaneous PET/MRI is a promising technology for the detection of neoplastic disease. There are inverse correlations between SUVmax and ADCmin and between SUV ratio and ADCmin. Correlation coefficients between SUVmax and ADCmin from PET/MRI were similar to values obtained with SUVmax from the same-day PET/CT. Given that both SUV and ADC are related to malignancy and that the correlation between the two biomarkers is relatively weak, SUV and ADC values may offer complementary information to aid in determination of prognosis and treatment response. The combined tumoral biomarker, ratio between SUVmax and ADCmin, may be useful for assessing progressive disease versus partial treatment response.

Comparison of the accuracy of PET/CT and PET/MRI spatial registration of multiple metastatic lesions.

OBJECTIVE The purpose of this study was to compare the accuracy of the spatial registration of conventional PET/CT with that of hybrid PET/MRI of patients with FDG-avid metastatic lesions. SUBJECTS AND METHODS Thirteen patients with known metastatic lesions underwent FDG PET/CT followed by PET/MRI with a hybrid whole-body system. The inclusion criterion for tumor analysis was spherical or oval FDG-avid tumor clearly identified with both CT and MRI. The spatial coordinates (x, y, z) of the visually estimated centers of the lesions were determined for PET/CT (PET and CT independently) and PET/MRI (PET, T1-weighted gradient-echo sequence with radial stack-of-stars trajectory, T2-weighted sequence), and the b0 images of an echo-planar imaging (EPI) diffusion-weighted imaging (DWI) acquisition. All MRI sequences were performed in the axial plane with free breathing. The spatial coordinates of the estimated centers of the lesions were determined for PET and CT and PET and MRI sequences. Distance between the isocenter of the lesion on PET images and on the images obtained with the anatomic modalities was measured, and misregistration (in millimeters) was calculated. The degree of misregistration was compared between PET/CT and PET/MRI with a paired Student t test. RESULTS Nineteen lesions were evaluated. On PET/CT images, the average of the total misregistration in all planes of CT compared with PET was 4.13 ± 4.24 mm. On PET/MR images, lesion misregistration between PET and T1-weighted gradient-echo images had a shift of 2.41 ± 1.38 mm and between PET and b0 DW images was 5.97 ± 2.83 mm. Similar results were calculated for 11 lesions on T2-weighted images. The shift on T2-weighted images compared with PET images was 2.24 ± 1.12 mm. Paired Student t test calculations for PET/CT compared with PET/MRI T1-weighted gradient-echo images with a radial stack-of-stars trajectory, b0 DW images, and T2-weighted images showed significant differences (p < 0.05). Similar results were seen in the analysis of six lung lesions. CONCLUSION PET/MRI T1-weighted gradient-echo images with a radial stack-of-stars trajectory and T2-weighted images had more accurate spatial registration than PET/CT images. This may be because that the whole-body PET/MRI system used can perform simultaneous acquisition, whereas the PET/CT system acquires data sequentially. However, the EPI-based b0 DWI datasets were significantly misregistered compared with the PET/CT datasets, especially in the thorax. Radiologists reading PET/MR images should be aware of the potential for misregistration on images obtained with EPI-based DWI sequences because of inherent spatial distortion associated with this type of MRI acquisition.

Peripheral arterial disease in a symptomatic diabetic population: prospective comparison of rapid unenhanced MR angiography (MRA) with contrast-enhanced MRA.

OBJECTIVE The joint guidelines of the American College of Cardiology and American Heart Association support the use of contrast-enhanced MR angiography (CEMRA) to diagnose the location and degree of stenosis in patients with known or suspected peripheral arterial disease (PAD). The high prevalence of chronic renal impairment in diabetic patients with PAD and the need for high doses of gadolinium-based contrast agents place them at risk for nephrogenic systemic fibrosis. The purpose of our study was to evaluate the accuracy of the rapid technique of quiescent-interval single-shot (QISS) unenhanced MR angiography (MRA) compared with CEMRA for the diagnosis in diabetic patients referred with symptomatic chronic PAD. SUBJECTS AND METHODS This prospective two-center study evaluated 25 consecutive diabetic patients with documented or suspected symptomatic PAD. Both centers used identical imaging protocols. Images were independently analyzed by two radiologists. A subgroup analysis was performed of patients who were also assessed with digital subtraction angiography (DSA) as part of the standard-of-care protocol before revascularization. RESULTS For this study, 775 segments were analyzed. On a per-segment basis, the mean values of the diagnostic accuracy of unenhanced MRA compared with reference CEMRA for two reviewers, reviewers 1 and 2, were as follows: sensitivity, 87.4% and 92.1%; specificity, 96.8% and 96.0%; positive predictive value, 90.8% and 94.0%; and negative predictive value, 95.5% and 94.6%. Substantial agreement was found when overall DSA results were compared with QISS unenhanced MRA (κ = 0.68) and CEMRA (κ = 0.63) in the subgroup of patients who also underwent DSA. There was almost perfect agreement between the two readers for stenosis scores, with Cohens kappa values being greater than 0.80 for both MRA techniques. CONCLUSION The results of our study indicate that QISS unenhanced MRA is an accurate noncontrast alternative to CEMRA for showing clinically significant arterial disease in patients with diabetes with symptomatic PAD.

Prospective Self-Gated Nonenhanced Magnetic Resonance Angiography of the Peripheral Arteries

Most nonenhanced MRA techniques for evaluating peripheral artery disease (PAD) require cardiac synchronization through physiological gating. Electrocardiographic gating is the most popular method for cardiac synchronization; however, it is subject to interference from switching magnetic field gradients and radiofrequency pulses. A method is described for self‐gated nonenhanced MRA that does not require the use of electrocardiographic gating. Imaging was prospectively triggered by detecting the acceleration of blood flow during systole with a reference‐less phase contrast navigator. The technique was implemented for nonsubtractive nonenhanced MRA using quiescent‐interval single‐shot MRA. The lower extremity peripheral arteries of eight healthy subjects were imaged using electrocardiographic‐, pulse‐, and self‐gated quiescent‐interval single‐shot. Self‐gated quiescent‐interval single‐shot triggered with 99% accuracy. There were no significant differences in relative contrast, contrast‐to‐noise ratio, or image quality between self‐gated and electrocardiographic‐gated quiescent‐interval single‐shot MRA (P > 0.05). Image quality with pulse gating was inferior. Magn Reson Med, 2013.

Comprehensive brain analysis with automated high-resolution magnetization transfer measurements

To enhance the reliability and spatial resolution of magnetization transfer ratio (MTR) measurements for interrogation of subcortical brain regions with an automated volume of interest (VOI) approach.

Assessing Low-frequency Repetitive Transcranial Magnetic Stimulation with Functional Magnetic Resonance Imaging: A Case Series

BACKGROUND AND PURPOSE This case series assesses the effects of five consecutive days of low-frequency (1 Hz) repetitive transcranial magnetic stimulation (rTMS) with and without a 6-Hz primer. Although this paper studies able-bodied individuals, similar rTMS protocols are used to facilitate motor recovery in patients with hemiplegia following stroke. However, the cortical mechanisms associated with repeated daily doses of rTMS are not completely understood. CASE DESCRIPTION Four right-handed healthy volunteers (two men, aged 20-50 years) participated in a double-blind case series of primed and unprimed rTMS. Functional magnetic resonance imaging was used to compare task-related haemodynamics during a simple motor task and resting-state cortical connectivity. Understanding the mechanisms of repeated rTMS sessions may serve as a precursor to development of rTMS paradigms involving motor cortex stimulation in patients with a range of neurologic dysfunction. OUTCOMES Following five consecutive days of rTMS, all subjects had reduced task-related haemodynamics. Resting-state brain connectivity between motor regions was reduced only after primed rTMS. DISCUSSION This is the first study to indicate that resting-state brain connectivity can distinguish the effect of primed and unprimed rTMS to a greater extent than task-related haemodynamics. Furthermore, priming may inhibit the connectivity between the area of the cortex underlying the rTMS site and remote brain regions. SIGNIFICANCE These findings benefit rTMS rehabilitation studies by examining haemodynamics on repeated days of stimulation and incorporating resting-state brain connectivity analysis to further understand underlying neural mechanisms. Furthermore, this work encourages the utilization of resting connectivity in future rTMS studies.

Optimization of single shot 3D breath-hold non-enhanced MR angiography of the renal arteries.

BackgroundCardiac and navigator-gated, inversion-prepared non-enhanced magnetic resonance angiography techniques can accurately depict the renal arteries without the need for contrast administration. However, the scan time and effectiveness of navigator-gated techniques depend on the subject respiratory pattern, which at times results in excessively prolonged scan times or suboptimal image quality. A single-shot 3D magnetization-prepared steady-state free precession technique was implemented to allow the full extent of the renal arteries to be depicted within a single breath-hold.MethodsTechnical optimization of the breath-hold technique was performed with fourteen healthy volunteers. An alternative magnetization preparation scheme was tested to maximize inflow signal. Quantitative and qualitative comparisons were made between the breath-hold technique and the clinically accepted navigator-gated technique in both volunteers and patients on a 1.5 T scanner.ResultsThe breath-hold technique provided an average of seven fold reduction in imaging time, without significant loss of image quality. Comparable single-to-noise and contrast-to-noise ratios of intra- and extra-renal arteries were found between the breath-hold and the navigator-gated techniques in volunteers. Furthermore, the breath-hold technique demonstrated good image quality for diagnostic purposes in a small number of patients in a pilot study.ConclusionsThe single-shot, breath-hold technique offers an alternative to navigator-gated methods for non-enhanced renal magnetic resonance angiography. The initial results suggest a potential supplementary clinical role for the breath-hold technique in the evaluation of suspected renal artery diseases.

Inline directionally independent peak velocity evaluation reduces error in peak antegrade velocity estimation in patients referred for cardiac valvular assessment.

OBJECTIVE The purpose of this article is to evaluate the utility of a tool in quantifying the peak antegrade velocity when assessing patients with cardiac valvular pathology. MATERIALS AND METHODS Directionally independent peak velocity evaluation (MaxVelocity, Siemens Healthcare) phase-contrast cardiac MRI was performed for 44 patients referred to our institution with a diagnosis or concern for aortic valvular disease or undergoing imaging for thoracic aortic aneurysm. In addition, standard through-plane phase-contrast MR angiography at the level of the aortic valve was performed. The MaxVelocity technique provides a simple tool to extract the magnitude of the peak velocity, independently of its direction, from phase-contrast imaging with velocity encoding. Recent echocardiography (within 1 month) and assessment of peak forward velocity at the level of the aortic valve were required for inclusion in the study. RESULTS The MaxVelocity technique shows significantly lower error in estimating peak antegrade velocity at the level of the aortic valve than does standard unidirectional through-plane phase-contrast MRI, using transthoracic echocardiography as the reference noninvasive imaging method. CONCLUSION Relative to standard through-plane imaging, MaxVelocity more closely approximates echocardiography for noninvasive assessment of peak antegrade velocity. Improved accuracy is critical for surgical decision making in patients with aortic valvular disease. Therefore, MaxVelocity provides an easy approach to quantify peak velocity as part of a routine clinical MRI protocol.

Simultaneous Acquisition of Gradient Echo / Spin Echo BOLD and Perfusion with a Separate Labeling Coil

Arterial spin labeling‐based cerebral blood flow imaging complements blood oxygenation level dependent (BOLD) imaging with a measure that is more quantitative and has better specificity to neuronal activation. Relative to gradient echo BOLD, spin echo BOLD has better spatial specificity because it is less biased to large draining veins. Although there have been many studies comparing simultaneously acquired cerebral blood flow data with gradient echo BOLD data in fMRI, there have been few studies comparing cerebral blood flow with SE BOLD and no study comparing all three. We present a pulse sequence that simultaneously acquires cerebral blood flow data with a separate labeling coil, gradient echo BOLD, and spin echo BOLD images. Simultaneous acquisition avoids interscan variability, allowing more direct assessment and comparison of each contrasts relative specificity and reproducibility. Furthermore, it facilitates studies that may benefit from multiple complementary measures. Magn Reson Med, 2010.