Improving image quality in transcranial magnetic resonance guided focused ultrasound using a conductive screen.

Transcranial Magnetic Resonance guided Focused Ultrasound (TcMRgFUS) has been proven to be an effective treatment for some neurological disorders such as essential and Parkinson's tremor. However, magnetic resonance guidance at 3 Tesla (3T) frequencies and using the large hemispherical transducers required for TcMRgFUS results in artifactual low-signal bands that pass through key regions of the brain. The purpose of this work was to investigate the use of a circular conductive Radio Frequency (RF) screen, that is bent to have a 12 cm radius in one direction and positioned near the top or back of the head, to reduce or remove these artifactual low-signal bands in TcMRgFUS. The impact of using an RF screen to remove these low signal bands was studied in both imaging experiments and electromagnetic simulations. Hydrophone measurements of the acoustic transparency of the bronze 2 mm diameter square mesh screen used in the imaging studies were compared with temperature measurements with and without the screen in heating studies in the TcMRgFUS system. The imaging and simulation studies both show that for the different screen configurations studied in this work, RF screen removes the low-signal bands and increases both homogeneity and signal-to-noise ratio (SNR) throughout the region of the brain. Hydrophone and heating studies indicate that even a 2 mm wire mesh provides minimal attenuation to the ultrasound beam. Simulation results also suggest that a 1 cm mesh will provide adequate artifact suppression with even less ultrasound attenuation. An RF screen that disrupts the natural waveguide nature of the transducer in the 3T MR environment can change the electromagnetic field profile to reduce unwanted artifacts and provide an imaging region which has more homogeneity and higher SNR throughout the brain.

Effect of Time Elapsed since Gadolinium Administration on Atherosclerotic Plaque Enhancement in Clinical Vessel Wall MR Imaging Studies.

Vessel wall MR imaging is a useful tool for the evaluation of intracranial atherosclerotic disease. Enhancement can be particularly instructive. This study investigated the impact of the duration between contrast administration and image acquisition. The cohort with the longest duration had the greatest increase in signal intensity change. When using vessel wall MR imaging to assess intracranial atherosclerotic disease, protocols should be designed to maximize the duration between contrast administration and image acquisition to best demonstrate enhancement.

Low-intensity ultrasound activates vestibular otolith organs through acoustic radiation force.

The present study examined the efficacy of 5 MHz low-intensity focused ultrasound (LiFU) as a stimulus to remotely activate inner ear vestibular otolith organs. The otolith organs are the primary sensory apparati responsible for detecting orientation of the head relative to gravity and linear acceleration in three-dimensional space. These organs also respond to loud sounds and vibration of the temporal bone. The oyster toadfish, Opsanus tau, was used to facilitate unobstructed acoustic access to the otolith organs in vivo. Single-unit responses to amplitude-modulated LiFU were recorded in afferent neurons identified as innervating the utricle or the saccule. Neural responses were equivalent to direct mechanical stimulation, and arose from the nonlinear acoustic radiation force acting on the otolithic mass. The magnitude of the acoustic radiation force acting on the otolith was measured ex vivo. Results demonstrate that LiFU stimuli can be tuned to mimic directional forces occurring naturally during physiological movements of the head, loud air conducted sound, or bone conducted vibration.

Sensitivity of tissue properties derived from MRgFUS temperature data to input errors and data inclusion criteria: ex vivo study in porcine muscle.

This study evaluates the sensitivity of two magnetic resonance-guided focused ultrasound (MRgFUS) thermal property estimation methods to errors in required inputs and different data inclusion criteria. Using ex vivo pork muscle MRgFUS data, sensitivities to required inputs are determined by introducing errors to ultrasound beam locations (r error = -2 to 2 mm) and time vectors (t error = -2.2 to 2.2 s). In addition, the sensitivity to user-defined data inclusion criteria is evaluated by choosing different spatial (r fit = 1-10 mm) and temporal (t fit = 8.8-61.6 s) regions for fitting. Beam location errors resulted in up to 50% change in property estimates with local minima occurring at r error = 0 and estimate errors less than 10% when r error < 0.5 mm. Errors in the time vector led to property estimate errors up to 40% and without local minimum, indicating the need to trigger ultrasound sonications with the MR image acquisition. Regarding the selection of data inclusion criteria, property estimates reached stable values (less than 5% change) when r fit > 2.5 × FWHM, and were most accurate with the least variability for longer t fit. Guidelines provided by this study highlight the importance of identifying required inputs and choosing appropriate data inclusion criteria for robust and accurate thermal property estimation. Applying these guidelines will prevent the introduction of biases and avoidable errors when utilizing these property estimation techniques for MRgFUS thermal modeling applications.

Vitamin D and Vulnerable Carotid Plaque.

BACKGROUND AND PURPOSE: MR imaging-detected carotid intraplaque hemorrhage indicates vulnerable plaque with high stroke risk. Angiotensin II stimulates intraplaque hemorrhage in animal models, and the angiotensin system is highly regulated by vitamin D. Our purpose was to determine whether low vitamin D levels predict carotid intraplaque hemorrhage in humans. MATERIALS AND METHODS: In this cross-sectional study, 65 patients with carotid disease underwent carotid MR imaging and blood draw. Systemic clinical confounders and local lumen imaging markers were recorded. To determine the association of low vitamin D levels with MR imaging detected intraplaque hemorrhage, we performed multivariable Poisson regression by using generalized estimating equations to account for up to 2 carotid arteries per patient and backward elimination of confounders. MR imaging detected intraplaque hemorrhage volume was also correlated with vitamin D levels and maximum plaque thickness. Thirty-five patients underwent carotid endarterectomy, and histology-detected intraplaque hemorrhage was correlated with vitamin D levels and total plaque area. RESULTS: Low vitamin D levels (<30 ng/mL, prevalence ratio = 2.05, P = .03) were a significant predictor of MR imaging detected intraplaque hemorrhage, along with plaque thickness (prevalence ratio = 1.40, P < .001). MR imaging detected intraplaque hemorrhage volume linearly correlated with plaque thickness (partial r = 0.45, P < .001) and low vitamin D levels (partial r = 0.26, P = .003). Additionally, histology-detected intraplaque hemorrhage area linearly correlated with plaque area (partial r = 0.46, P < .001) and low vitamin D levels (partial r = 0.22, P = .03). The association of intraplaque hemorrhage volume with low vitamin D levels was also higher with ischemic stroke. CONCLUSIONS: Low vitamin D levels and plaque thickness predict carotid intraplaque hemorrhage and outperform lumen markers of vulnerable plaque. This research demonstrates a significant link between low vitamin D levels and carotid intraplaque hemorrhage.

Prediction of Carotid Intraplaque Hemorrhage Using Adventitial Calcification and Plaque Thickness on CTA.

BACKGROUND AND PURPOSE: Carotid intraplaque hemorrhage is associated with stroke, plaque thickness, stenosis, ulceration, and adventitial inflammation. Conflicting data exist on whether calcification is a marker of plaque instability, and no data exist on adventitial calcification. Our goal was to determine whether adventitial calcification and soft plaque (a rim sign) help predict carotid intraplaque hemorrhage. MATERIALS AND METHODS: This was a retrospective cohort study of 96 patients who underwent carotid MRA and CTA within 1 month, from 2009 to 2016. We excluded occlusions (n = 4) and near occlusions (n = 0), leaving 188 carotid arteries. Intraplaque hemorrhage was detected by using MPRAGE. Calcification, adventitial pattern, stenosis, maximum plaque thickness (total, soft, and hard), ulceration, and intraluminal thrombus on CTA were recorded. Atherosclerosis risk factors and medications were recorded. We used mixed-effects multivariable Poisson regression, accounting for 2 vessels per patient. For the final model, backward elimination was used with a threshold of P < .10. Receiver operating characteristic analysis determined intraplaque hemorrhage by using the area under the curve. RESULTS: Our final model included the rim sign (prevalence ratio = 11.9, P < .001) and maximum soft-plaque thickness (prevalence ratio = 1.2, P = .06). This model had excellent intraplaque hemorrhage prediction (area under the curve = 0.94), outperforming the rim sign, maximum soft-plaque thickness, NASCET stenosis, and ulceration (area under the curve = 0.88, 0.86, 0.77, and 0.63, respectively; P < .001). Addition of the rim sign performed better than each marker alone, including maximum soft-plaque thickness (area under the curve = 0.94 versus 0.86, P < .001), NASCET stenosis (area under the curve = 0.90 versus 0.77, P < .001), and ulceration (area under the curve = 0.90 versus 0.63, P < .001). CONCLUSIONS: The CTA rim sign of adventitial calcification with internal soft plaque is highly predictive of carotid intraplaque hemorrhage.

Optimal Prediction of Carotid Intraplaque Hemorrhage Using Clinical and Lumen Imaging Markers.

BACKGROUND AND PURPOSE: MR imaging detects intraplaque hemorrhage with high accuracy by using the magnetization-prepared rapid acquisition of gradient echo sequence. Still, MR imaging is not readily available for all patients, and many undergo CTA instead. Our goal was to determine essential clinical and lumen imaging predictors of intraplaque hemorrhage, as indicators of its presence and clues to its pathogenesis. MATERIALS AND METHODS: In this retrospective cross-sectional study, patients undergoing stroke work-up with MR imaging/MRA underwent carotid intraplaque hemorrhage imaging. We analyzed 726 carotid plaques, excluding vessels with non-carotid stroke sources (n = 420), occlusions (n = 7), or near-occlusions (n = 3). Potential carotid imaging predictors of intraplaque hemorrhage included percentage diameter and millimeter stenosis, plaque thickness, ulceration, and intraluminal thrombus. Clinical predictors were recorded, and a multivariable logistic regression model was fitted. Backward elimination was used to determine essential intraplaque hemorrhage predictors with a thresholded 2-sided P < .10. Receiver operating characteristic analysis was also performed. RESULTS: Predictors of carotid intraplaque hemorrhage included plaque thickness (OR = 2.20, P < .001), millimeter stenosis (OR = 0.46, P < .001), ulceration (OR = 4.25, P = .020), age (OR = 1.11, P = .001), and male sex (OR = 3.23, P = .077). The final model discriminatory value was excellent (area under the curve = 0.932). This was significantly higher than models using only plaque thickness (area under the curve = 0.881), millimeter stenosis (area under the curve = 0.830), or ulceration (area under the curve= 0.715, P < .001). CONCLUSIONS: Optimal discrimination of carotid intraplaque hemorrhage requires information on plaque thickness, millimeter stenosis, ulceration, age, and male sex. These factors predict intraplaque hemorrhage with high discriminatory power and may provide clues to the pathogenesis of intraplaque hemorrhage. This model could be used to predict the presence of intraplaque hemorrhage when MR imaging is contraindicated.

Workplace safety and health programs, practices, and conditions in auto collision repair businesses.

This article describes the results of a pre-intervention safety assessment conducted in 49 auto collision repair businesses and owners' commitments to specific improvements. A 92-item standardized audit tool employed interviews, record reviews, and observations to assess safety and health programs, training, and workplace conditions. Owners were asked to improve at least one-third of incorrect, deficient, or missing (not in compliance with regulations or not meeting best practice) items, of which a majority were critical or highly important for ensuring workplace safety. Two-thirds of all items were present, with the highest fraction related to electrical safety, machine safety, and lockout/tagout. One-half of shops did not have written safety programs and had not conducted recent training. Many had deficiencies in respiratory protection programs and practices. Thirteen businesses with a current or past relationship with a safety consultant had a significantly higher fraction of correct items, in particular related to safety programs, up-to-date training, paint booth and mixing room conditions, electrical safety, and respiratory protection. Owners selected an average of 58% of recommended improvements; they were most likely to select items related to employee Right-to-Know training, emergency exits, fire extinguishers, and respiratory protection. They were least likely to say they would improve written safety programs, stop routine spraying outside the booth, or provide adequate fire protection for spray areas outside the booth. These baseline results suggest that it may be possible to bring about workplace improvements using targeted assistance from occupational health and safety professionals.

Effects of MRTI sampling characteristics on estimation of HIFU SAR and tissue thermal diffusivity.

While the non-invasive and three-dimensional nature of magnetic-resonance temperature imaging (MRTI) makes it a valuable tool for high-intensity focused ultrasound (HIFU) treatments, random and systematic errors in MRTI measurements may propagate into temperature-based parameter estimates used for pretreatment planning. This study assesses the MRTI effects of zero-mean Gaussian noise (SD = 0.0-2.0 °C), temporal sampling (tacq = 1.0-8.0 s), and spatial averaging (Res = 0.5-2.0 mm isotropic) on HIFU temperature measurements and temperature-based estimates of the amplitude and full width half maximum (FWHM) of the HIFU specific absorption rate and of tissue thermal diffusivity. The ultrasound beam used in simulations and ex vivo pork loin experiments has lateral and axial FWHM dimensions of 1.4 mm and 7.9 mm respectively. For spatial averaging simulations, beams with lateral FWHM varying from 1.2-2.2 mm are also assessed. Under noisy conditions, parameter estimates are improved by fitting to data from larger voxel regions. Varying the temporal sampling results in minimal changes in measured temperatures (<2% change) and parameter estimates (<5% change). For the HIFU beams studied, a spatial resolution of 1 × 1 × 3 mm(3) or smaller is required to keep errors in temperature and all estimated parameters less than 10%. By quantifying the errors associated with these sampling characteristics, this work provides researchers with appropriate MRTI conditions for obtaining estimates of parameters essential to pretreatment modeling of HIFU thermal therapies.

In vivo evaluation of a breast-specific magnetic resonance guided focused ultrasound system in a goat udder model.

PURPOSE: This work further evaluates the functionality, efficacy, and safety of a new breast-specific magnetic resonance guided high intensity focused ultrasound (MRgFUS) system in an in vivo goat udder model. METHODS: Eight female goats underwent an MRgFUS ablation procedure using the breast-specific MRgFUS system. Tissue classification was achieved through the 3D magnetic resonance imaging (MRI) acquisition of several contrasts (T1w, T2w, PDw, 3-point Dixon). The MRgFUS treatment was performed with a grid trajectory executed in one or two planes within the glandular tissue of the goat udder. Temperature was monitored using a 3D proton resonance frequency (PRF) MRI technique. Delayed contrast enhanced-MR images were acquired immediately and 14 days post MRgFUS treatment. A localized tissue excision was performed in one animal and histological analysis was performed. Animals were available for adoption at the conclusion of the study. RESULTS: The breast-specific MRgFUS system was able to ablate regions ranging in size from 0.4 to 3.6 cm(3) in the goat udder model. Tissue damage was confirmed through the correlation of thermal dose measurements obtained with realtime 3D MR thermometry to delayed contrast enhanced-MR images immediately after the treatment and 14 days postablation. In general, lesions were longer in the ultrasound propagation direction, which is consistent with the dimensions of the ultrasound focal spot. Thermal dose volumes had better agreement with nonenhancing areas of the DCE-MRI images obtained 14 days after the MRgFUS treatment. CONCLUSIONS: The system was able to successfully ablate lesions up to 3.6 cm(3). The thermal dose volume was found to correlate better with the 14-day postablation nonenhancing delayed contrast enhanced-MR image volumes. While the goat udder is not an ideal model for the human breast, this study has proven the feasibility of using this system on a wide variety of udder shapes and sizes, demonstrating the flexibility that would be required in order to treat human subjects.