No abstract

We evaluated radiofrequency (RF) heating of various implants embedded in a gel phantom during magnetic resonance (MR) procedures. We examined the dependence of RF heating on variation in speciˆc absorption rate (SAR) and angle between the implant and the static magneticˆeld (B 0 ) and on the displacement of the phantom in the irradiation coil using a 1.5-tesla MR system, and we compared the in‰uence of RF heating on the same implant using a 3.0T MR system.Our results support the occurrence of RF heating of implants made of non-magnetizing metal. We observed greater RF heating when the implant was set parallel to B 0 , embedded at a shallower depth, and placed at the center of the RF irradiation coil. We also conˆrmed that the rise in temperature was proportionate to the increase in SAR. We considered the diŠerence in temperature elevation on depth of embedding to re‰ect the skin-depth eŠect of RF intensity for both the 1.5-and 3.0-T MR systems.

Purpose: We evaluate radiofrequency (RF) heating of two kinds of hip joint implants of different sizes, shapes and materials. Temperature rises at various positions of each implant are measured and compared with a computer simulation based on electromagnetic-field analysis. Methods: Two kinds of implants made of cobalt-chromium alloy and titanium alloy were embedded at a 2-cm depth of tissue-equivalent gel-phantom. The phantom was placed parallel to the static magnetic field of a 1.5 T MRI device. Scans were conducted at the specific absorption rate of 2.5 W/kg for 15 min, and temperatures were recorded with RF-transparent fiberoptic sensors. Temperatures of the implant surface were measured at 6 positions, from the tip to the head. Measured temperature rises were compared with the results of electromagnetic-field analysis. Results: The maximum temperature rise was observed at the tip of each implant, and it was 9.0˚C for the cobaltchromium implant and 5.3˚C for the titanium implant. The simulated heating positions with electromagneticfield analysis accorded with experimental results. However, a difference in temperature rise was seen with the titanium implant. Conclusion: RF heating was confirmed to take place at both ends of the implants in spite of their different shapes. The maximum temperature rise was observed at the tip where there is large curvature. The value was found to depend on physical properties of the implant materials. The discrepancy between experimental and simulated temperature rises was presumed to be the result of an incomplete model for the titanium implant.

Chemical exchange saturation transfer (CEST) is a new contrast enhancement approach for imaging exogenous or endogenous substances such as creatine (Cr), amide protons, and glutamate in the human body. An increase in field strength is beneficial for CEST imaging because of the increased chemical shift and longer longitudinal relaxation time (T1). In high-field magnetic resonance imaging (MRI), establishing and evaluating the CEST effect is important for optimizing the magnetization transfer (MT) saturation radio frequency (RF) pulses. In this study, the CEST effect on Cr was evaluated at different concentrations in pH phantoms by appropriately selecting MT saturation RF pulses using 11.7 T MRI. The results showed that the CEST efficiency increased gradually with increasing applied saturation RF pulse power and that it was affected by the number of saturation RF pulses and their bandwidths. However, spillover effects were observed with higher saturation RF pulse powers. In conclusion, we successfully performed in vitro Cr CEST imaging under optimized conditions of MT saturation RF pulses.

Increased x-ray exposure to physicians’ eye lenses during radiology procedures is a significant concern. In this study, x-ray exposure to the eye was measured using an anthropomorphic head phantom, with and without radiation-protective devices, to examine the dose of x-ray radiation that physicians are exposed to during endoscopic retrograde cholangiopancreatography (ERCP). X-ray exposure of the eye was measured using novel dedicated direct eye lens dosimeters that could specifically measure H p(3) during the ERCP procedure. The spatial dose in the height direction of the physician was measured using an ionization chamber dosimeter. Eye dosimeters were attached inside and outside the lead (Pb) glasses attached to the head of the human phantom to demonstrate its protective effect. Irradiation from the system lasted for 30 min. When the overcouch x-ray tube system is used, the cumulative radiation dose over the 30 min x-ray fluoroscopy time, without the use of radiation-protective devices, to the left and right eyes was 3.7 and 1.5 mSv, respectively. This dose was estimated to be the dose to the lens per therapeutic ERCP examination. With radiation-protective glasses, the dose reduced to 1.8 and 1.0 mSv for the left and right eye, respectively. The results of our study indicated that radiation exposure to the eye was reduced by up to 80.0% using Pb glasses and by 96.8% using radiation-protective curtains. Our study indicates that a physician’s maximum radiation exposure to the eyes during an ERCP procedure may be above the level recommended by the International Commission on Radiological Protection when the physician does not use radiation-protective devices. The eyewear, which is larger and fitted more closely to the face, provided a better protection effect even with a low lead equivalence, demonstrating that the shape of eyewear is important for protective function.