[Validity of the Signal-to-noise Ratio Measurement Method for MRI Using a Phase Image].

The most common methods to determine the signal-to-noise ratio (SNR) of magnetic resonance imaging (MRI) are based on the signal statistics in regions of interest (ROIs) in a magnitude image. For this, methods to calculate the ROI have still several discussions; we assumed SNR of a magnitude image could be estimated from standard deviation of a phase image (the phase method). The purpose of this study is to evaluate the validity of the phase method to determine the SNR. The simulation using digital phantom was carried out for evaluation of the effect to measure SNR; fluctuation in the phase image and SNR of the magnitude image. The phantom study was also performed for evaluation of the validity of estimation using the phase method in comparison with the conventional method (the identical ROI method and the subtraction method). The result of the simulation showed that SNR of magnitude image is larger than 4 for the SNR measurement using the phase method and this results reliable. The influence of fluctuation of the phase image should be eliminated for practical purposes. In the phantom study, phase method showed similar results compared to conventional methods in condition with elimination of the fluctuation of the phase image. Though there was a difference in the results of the phase method and the subtraction method according to the position of the ROI, the error was less than 4%. In this study, the method using the phase image to determine the SNR was identified as valuable.

Preliminary experience with superparamagnetic iron oxide-enhanced dynamic magnetic resonance imaging and comparison with contrast-enhanced computed tomography in endoleak detection after endovascular aneurysm repair.

OBJECTIVE: Contrast-enhanced computed tomography (CE-CT) has been commonly used for follow-up imaging after endovascular aneurysm repair (EVAR), but it is difficult to use on patients with renal insufficiency. Superparamagnetic iron oxide (SPIO) particles, contrast medium for magnetic resonance imaging (MRI) that has been widely used for detection of the liver tumor, rarely affects renal function. The present study examined SPIO-enhanced dynamic MRI as a potential alternative to CE-CT for detection of endoleaks after EVAR. METHODS: Institutional review board approval was obtained for this prospective study. Twenty-three consecutive patients with normal renal function were evaluated using both CE-CT and SPIO-enhanced MRI within 2 weeks after EVAR. The median interval between the two modalities was 2 days. SPIO-enhanced MRI was performed at 1.5 T with T1-weighted, SPIO-enhanced dynamic, and postcontrast T1-weighted gradient echo sequences. The CE-CT protocol consisted of triple scans. Two experienced, blinded observers evaluated all images. Consensus reading of CE-CT and SPIO-MRI was defined as the reference standard. Interobserver, intraobserver, and intermodality agreement for endoleak detection was assessed by κ statistics. RESULTS: A total of 11 type II endoleaks originating from either the lumbar or inferior mesenteric artery were detected. Eight were able to be detected by CE-CT (8/11:73%) and 10 (10/11:91%) by SPIO-enhanced MRI. Interobserver (κ = 0.91; 95% CI, 0.74-1.00) and intraobserver agreement for MRI (κ = 1.00) were excellent. Intermodality agreement for endoleak detection was moderate (κ = 0.63; 95% CI, 0.32-0.94; and κ = 0.62; 95% CI, 0.29-0.95 for observers A and B, respectively). CONCLUSIONS: SPIO-enhanced MRI could represent a useful alternative to CE-CT, as it offers excellent interobserver, intraobserver agreement, and could detect more endoleaks than CE-CT.

[The effect of the surroundings to the apparent diffusion coefficient on diffusion weighted imaging].

Diffusion weighted imaging (DWI) is now widely used in magnetic resonance (MR) imaging of the head and body. Moreover, the Apparent diffusion coefficient (ADC) value is often used for the differential diagnosis of the tumor. However, the effect of the surroundings on the ADC value has not been reported. In this study, we used the phantom completely sealed up to measure the change in the ADC value depending on the surroundings material. The results showed that the ADC value decreased according to the density of superparamagnetic iron oxide (SPIO) in the surroundings. Clinically, hemorrhage or iron deposit around the tumor may affect the ADC value of the tumor and result in under-estimation.

[Evaluation of appearance with high-speed CE-3dMRDSA in brain].

We evaluated the fitting scan technique for CE-3dMRDSA, and found common ground that determines spatial resolution and time resolution (slab thickness and partition number). We also examined the relation between appearance and time resolution (volume of contrast medium and injection speed). To obtain good image contrast, the volume of contrast medium needs to be at least 7 ml and suitable for an injection speed of 3-5 ml/sec. However, when we increased the volume of contrast medium and decreased injection speed, changes in MRDSA images with time became worse. The measure of the bolus with contrast medium was found to determine image contrast. When contrast medium is injected earlier, it circulates earlier within the brain. If the scan time is not short enough, it is not possible to observe changes in MRDSA images. And when spatial resolution is improved, time resolution becomes worse. Therefore, it is important to find the point of compromise between spatial resolution and time resolution. If we look for anterior MIP images, the CNR in the spatial resolution didn't change, when the slice thickness is more than 3 mm. Because, the partial volume effect decide the image contrast. However, unless the view is from the front, slice thickness influences spatial resolution. Therefore, when we view MIP images from the lateral direction, slice thickness must be set at less than 2 mm. Results indicated that, in CE-3dMRDSA with the fitting technique, slice thickness should be less than 3 mm, partition number 16-20, slab thickness 48 mm, contrast medium volume 7-10 ml, and injection speed 3-5 ml.