The purpose was to evaluate radiofrequency (RF)-related heating of commonly used extracranial neu... more The purpose was to evaluate radiofrequency (RF)-related heating of commonly used extracranial neurosurgical implants in 7-T magnetic resonance imaging (MRI). Experiments were performed using a 7-T MR system equipped with a transmit/receive RF head coil. Four commonly used titanium neurosurgical implants were studied using a test procedure adapted from the American Society for Testing and Materials Standard F2182-11a. Implants (n=4) were tested with an MRI turbo spin echo pulse sequence designed to achieve maximum RF exposure [specific absorption rate (SAR) level=9.9W/kg], which was further validated by performing calorimetry. Maximum temperature increases near each implant's surface were measured using fiberoptic temperature probes in a gelled-saline-filled phantom that mimicked the conductive properties of soft tissue. Measurement results were compared to literature data for patient safety. The highest achievable phantom averaged SAR was determined by calorimetry to be 2.0±0.1W/kg due to the highly conservative SAR estimation model used by this 7-T MR system. The maximum temperature increase at this SAR level was below 1.0°C for all extracranial neurosurgical implants that underwent testing. The findings indicated that RF-related heating under the conditions used in this investigation is not a significant safety concern for patients with the particular extracranial neurosurgical implants evaluated in this study.
To compare the intraindividual differences of dynamic signal characteristics for 3 gadolinium che... more To compare the intraindividual differences of dynamic signal characteristics for 3 gadolinium chelates (gadopentetate dimeglumine ͓Gd-DTPA͔, gadodiamide ͓Gd-DTPA-BMA͔, and gadobenate dimeglumine ͓Gd-BOPTA͔) using dynamic contrast enhanced magnetic resonance (MR) imaging (DCE-MRI) with a preclinical beagle model at 7 Tesla. Method and Materials: Seven beagles were scanned 3 times each with a 7-day interval between the scans on a 7T whole body MRI system (Achieva, Philips) using a T/R head coil. Three different Gd contrast agents including Gd-DTPA, Gd-DTPABMA, and Gd-BOPTA were injected in a randomized order with a power injector (Spectris, MedRad, Indianola, PA) using dose of 0.1 mmol/kg body weight and flow rate of 0.06 mL/s. During image acquisition and data analysis, the identity of the specific contrast agent used for each examination was blinded. A 3D RF-spoiled fast field echo sequence was used for dynamic scans with in-plane spatial resolution 0.47 ϫ 0.47 mm 2 , temporal resolution of 9.5 seconds, and a total of 60 time points. Regions of interest were drawn within the carotid arteries and muscle tissue to determine semiquantitative parameters including maximum enhancement ratio, area under the signal enhancement curve over 90 seconds after contrast injection (AUC_90), time to maximum signal enhancement (T max ), and washout_score. Additionally, quantitative pharmacokinetic parameters were measured in muscle tissues by applying 3 separate 2-compartment models;
One of the most powerful features of the dynamic contrastenhanced (DCE) MRI technique is its capa... more One of the most powerful features of the dynamic contrastenhanced (DCE) MRI technique is its capability to quantitatively measure the physiological or pathophysiological environments assessed by the passage of contrast agent by means of modelbased pharmacokinetic analysis. The widely used two-compartment pharmacokinetic model developed by Brix and colleges fits tumor data well in most cases, but fails to explain the biexponential arterial input function. In this work, this problem has been attacked from a theoretical point of view, showing that this problem can be solved by adopting a more realistic model assumption when simplifying the general solutions of the two-compartment pharmacokinetic equations. Pharmacokinetic parameters derived from our model were demonstrated to have comparative tissue specificity to K trans from Larsson's model, better than those from Brix's model and the empirical area-under-the-curve (AUC). Tissue-type classifier constructed with the arterial input function-decomposed k ep -k pe pair from our model was also demonstrated to have superior performance than any other classifier based on DCE-MRI pharmacokinetic parameters or empirical AUC. The feature that this classifier has a near-zero false-negative rate makes it a highly desirable tool for clinical diagnostic and response assessment applications.
The purpose was to evaluate radiofrequency (RF)-related heating of commonly used extracranial neu... more The purpose was to evaluate radiofrequency (RF)-related heating of commonly used extracranial neurosurgical implants in 7-T magnetic resonance imaging (MRI). Experiments were performed using a 7-T MR system equipped with a transmit/receive RF head coil. Four commonly used titanium neurosurgical implants were studied using a test procedure adapted from the American Society for Testing and Materials Standard F2182-11a. Implants (n=4) were tested with an MRI turbo spin echo pulse sequence designed to achieve maximum RF exposure [specific absorption rate (SAR) level=9.9W/kg], which was further validated by performing calorimetry. Maximum temperature increases near each implant's surface were measured using fiberoptic temperature probes in a gelled-saline-filled phantom that mimicked the conductive properties of soft tissue. Measurement results were compared to literature data for patient safety. The highest achievable phantom averaged SAR was determined by calorimetry to be 2.0±0.1W/kg due to the highly conservative SAR estimation model used by this 7-T MR system. The maximum temperature increase at this SAR level was below 1.0°C for all extracranial neurosurgical implants that underwent testing. The findings indicated that RF-related heating under the conditions used in this investigation is not a significant safety concern for patients with the particular extracranial neurosurgical implants evaluated in this study.
To compare the intraindividual differences of dynamic signal characteristics for 3 gadolinium che... more To compare the intraindividual differences of dynamic signal characteristics for 3 gadolinium chelates (gadopentetate dimeglumine ͓Gd-DTPA͔, gadodiamide ͓Gd-DTPA-BMA͔, and gadobenate dimeglumine ͓Gd-BOPTA͔) using dynamic contrast enhanced magnetic resonance (MR) imaging (DCE-MRI) with a preclinical beagle model at 7 Tesla. Method and Materials: Seven beagles were scanned 3 times each with a 7-day interval between the scans on a 7T whole body MRI system (Achieva, Philips) using a T/R head coil. Three different Gd contrast agents including Gd-DTPA, Gd-DTPABMA, and Gd-BOPTA were injected in a randomized order with a power injector (Spectris, MedRad, Indianola, PA) using dose of 0.1 mmol/kg body weight and flow rate of 0.06 mL/s. During image acquisition and data analysis, the identity of the specific contrast agent used for each examination was blinded. A 3D RF-spoiled fast field echo sequence was used for dynamic scans with in-plane spatial resolution 0.47 ϫ 0.47 mm 2 , temporal resolution of 9.5 seconds, and a total of 60 time points. Regions of interest were drawn within the carotid arteries and muscle tissue to determine semiquantitative parameters including maximum enhancement ratio, area under the signal enhancement curve over 90 seconds after contrast injection (AUC_90), time to maximum signal enhancement (T max ), and washout_score. Additionally, quantitative pharmacokinetic parameters were measured in muscle tissues by applying 3 separate 2-compartment models;
One of the most powerful features of the dynamic contrastenhanced (DCE) MRI technique is its capa... more One of the most powerful features of the dynamic contrastenhanced (DCE) MRI technique is its capability to quantitatively measure the physiological or pathophysiological environments assessed by the passage of contrast agent by means of modelbased pharmacokinetic analysis. The widely used two-compartment pharmacokinetic model developed by Brix and colleges fits tumor data well in most cases, but fails to explain the biexponential arterial input function. In this work, this problem has been attacked from a theoretical point of view, showing that this problem can be solved by adopting a more realistic model assumption when simplifying the general solutions of the two-compartment pharmacokinetic equations. Pharmacokinetic parameters derived from our model were demonstrated to have comparative tissue specificity to K trans from Larsson's model, better than those from Brix's model and the empirical area-under-the-curve (AUC). Tissue-type classifier constructed with the arterial input function-decomposed k ep -k pe pair from our model was also demonstrated to have superior performance than any other classifier based on DCE-MRI pharmacokinetic parameters or empirical AUC. The feature that this classifier has a near-zero false-negative rate makes it a highly desirable tool for clinical diagnostic and response assessment applications.
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Papers by Xiangyu Yang