Apparent Diffusion Coefficient of Normal Abdominal Organs and Bone Marrow From Whole-Body DWI at 1.5 T: The Effect of Sex and Age.

OBJECTIVE: The objectives of this study were to define the range of apparent diffusion coefficients (ADCs) from whole-body DWI in normal abdominal organs and bone marrow, to identify ADC differences between sexes and changes occurring with age, and to evaluate the effect of the fat fraction (FF) on the ADC of normal liver parenchyma and bone marrow. MATERIALS AND METHODS: Fifty-one healthy volunteers (mean age = 38 years; age range = 23-68 years) underwent whole-body DWI using single-shot echo-planar imaging (b = 0, 150, 400, 750, and 1000 s/mm(2)). A two-point Dixon technique was used to evaluate the FF. Perfusion-sensitive ADCs, which we refer to as "ADCALL," and perfusion-insensitive ADCs, which we refer to as "ADCHIGH," of the liver and renal parenchyma, spleen, pancreatic tail, and red and yellow bone marrow were calculated. The relationships between ADC and sex, age, and FF were examined. RESULTS: ADCALL and ADCHIGH were significantly higher in female volunteers for the pancreatic tail (p = 0.046 and 0.008, respectively), red bone marrow (p = 0.029 and 0.001), and yellow bone marrow (p < 0.001 for both) but with considerable overlap. There were significant negative correlations between ADCALL and ADCHIGH and age in the liver parenchyma (p = 0.008 and 0.01, respectively) and in the yellow bone marrow (p = 0.013 and 0.039) for all subjects. ADCALL and ADCHIGH were also negatively correlated with FF in the liver parenchyma (p = 0.006 and 0.008, respectively) and in yellow bone marrow (p < 0.001 and p = 0.001) in all subjects. CONCLUSION: The ADCs of normal liver parenchyma and bone marrow change significantly with age. The ADCs of bone marrow in women are significantly higher than those of men and correlate strongly with FF. These effects may have an impact on image interpretation when using whole-body DWI to assess disease burden and treatment response.

A phantom for diffusion-weighted MRI (DW-MRI).

PURPOSE: To develop tissue-equivalent diffusivity materials and build a spherical diffusion phantom which mimics the conditions typically found in biological tissues. Also, to assess the reproducibility of ADC measurements from a whole-body diffusion protocol. MATERIALS AND METHODS: Nickel-doped agarose/sucrose gels were manufactured and used to build a spherical diffusion phantom with tissue-equivalent relaxation and diffusion compartments. The temporal stability of the gels was monitored for a period of 8 weeks and, using the same measurements, the reproducibility of ADC was assessed in a 1.5 Tesla (T) clinical system. RESULTS: The temporal stability of the nickel-doped agarose/sucrose gels diffusion properties was excellent (average coefficient of variation [CV] for ADC in all phantom compartments = 1.27%). The average CV for ADC measurements, excluding the phantom compartments affected by artifacts, was 0.76% showing that the reproducibility of ADC measurements using an EPI DW-MRI protocol is very good. CONCLUSION: Nickel-doped agarose/sucrose gels can be used as reference materials for MRI diffusion measurements and show excellent short-term stability with respect to ADC. A phantom made of these materials can be invaluable in optimizing DW-MRI protocols, developing novel pulse sequences for DW-MRI, or comparing ADC values among field strengths, vendors, and imaging centers.

Numerical simulation of SAR induced around Co-Cr-Mo hip prostheses in situ exposed to RF fields associated with 1.5 and 3 T MRI body coils.

When patients with metallic prosthetic implants undergo an MR procedure, the interaction between the RF field and the prosthetic device may lead to an increase in specific absorption rate (SAR) in tissues surrounding the prosthesis. In this work, the distribution of SAR(10g) around bilateral CoCrMo alloy hip prostheses in situ in anatomically realistic voxel models of an adult male and female due to RF fields from a generic birdcage coil driven at 64 or 128 MHz are predicted using a time-domain finite integration technique. Results indicate that the spatial distribution and maximum values of SAR(10g) are dependent on body model, frequency, and the position of the coil relative to the body. Enhancement of SAR(10g) close to the extremities of a prosthesis is predicted. Values of SAR(10g) close to the prostheses are compliant with recommended limits if the prostheses are located outside the coil. However, caution is required when the prostheses are within the coil since the predicted SAR(10g) close to an extremity of a prosthesis exceeds recommended limits when the whole body averaged SAR is 2 W kg(-1) . Compliance with recommended limits is likely to require a reduction in the time averaged input power.