Small bowel imaging with MRI

Bowel tracking for MR-guided radiotherapy: simultaneous optimization of small bowel imaging and tracking

Objective. The small bowel is one of the most radiosensitive organs-at-risk during radiotherapy in the pelvis. This is further complicated due to anatomical and physiological motion. Thus, its accurate tracking becomes of particular importance during therapy delivery, to obtain better dose-toxicity relations and/or to perform safe adaptive treatments. The aim of this work is to simultaneously optimize the MR imaging sequence and motion estimation solution towards improved small bowel tracking precision during radiotherapy delivery.Approach. An MRI sequence was optimized, to adhere to the respiratory and peristaltic motion frequencies, by assesing the performance of an image registration algorithm on data acquired on volunteers and patients. In terms of tracking, three registration algorithms, previously-employed in the scope of image-guided radiotherapy, were investigated and optimized. The optimized scan was acquired for 7.5 min, in 18 patients and for 15 min, in 10 volunteers at a 1.5 T MRL (Unity, Elekta AB). The tracking precision was evaluated and validated by means of three different quality assurance criteria: Structural Similarity Index Measure (SSIM), Inverse Consistency (IC) and Absolute Intensity Difference.Main results. The optimal sequence was a balanced Fast Field Echo, which acquired a 3D volume of the abdomen, with a dynamic scan time of 1.8 s. An optical flow algorithm performed best and which was able to resolve most of the motion. This was shown by mean IC values of<1 mm and a mean SSIM>0.9for the majority of the cases. A strong positive correlation (p <0.001) between the registration performance and visceral fat percentage was found, where a higher visceral fat percentage gave a better registration due to the better image contrast.Significance. A method for simultaneous optimization of imaging and tracking was presented, which derived an imaging and registration procedure for accurate small bowel tracking on the MR-Linac.

Dotted Core-Shell Nanoparticles for T1 -Weighted MRI of Tumors

Gd-based T 1 -weighted contrast agents have dominated the magnetic resonance imaging (MRI) contrast agent market for decades. Nevertheless, they are reported to be nephrotoxic and the U.S. Food and Drug Administration has issued a general warning concerning their use. In order to reduce the risk of nephrotoxicity, the MRI performance of the Gd-based T 1 -weighted contrast agents needs to be improved to allow a much lower dosage. In this study, novel dotted core-shell nanoparticles (FeGd-HN3-RGD2) with superhigh r 1 value (70.0 mM-1 s-1 ) and very low r 2 /r 1 ratio (1.98) are developed for high-contrast T 1 -weighted MRI of tumors. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and histological analyses show good biocompatibility of FeGd-HN3-RGD2. Laser scanning confocal microscopy images and flow cytometry demonstrate active targeting to integrin αv β3 positive tumors. MRI of tumors shows high tumor ΔSNR for FeGd-HN3-RGD2 (477 ± 44%), which is about 6-7-fold higher than that of Magnevist (75 ± 11%). MRI and inductively coupled plasma results further confirm that the accumulation of FeGd-HN3-RGD2 in tumors is higher than liver and spleen due to the RGD2 targeting and small hydrodynamic particle size (8.5 nm), and FeGd-HN3-RGD2 is readily cleared from the body by renal excretion.

Gastrointestinal imaging-practical magnetic resonance imaging approach

Over the past two decades, advances in cross-sectional imaging such as computed tomography and magnetic resonance imaging (MRI) have dramatically changed the concept of gastrointestinal imaging. MR is playing an increasing role in the evaluation of gastrointestinal disorders. MRI combines the advantages of excellent soft-tissue contrast, noninvasiveness, functional information and lack of ionizing radiation. Furthermore, recent developments of MRI have led to improved spatial and temporal resolution as well as decreased motion artifacts. In this article we describe the technical aspects of gastrointestinal MRI and present a practical approach for a well-known spectrum of gastrointestinal disease processes.

A prospective randomized controlled study of erythromycin on gastric and small intestinal distention: implications for MR enterography

OBJECTIVES

To assess if erythromycin increases gastric emptying and hence improves small intestinal distention during MR enterography.

METHODS

Gastric, small intestinal, and large intestinal volumes were assessed with MR after neutral oral contrast (1350ml in 45min) and balanced randomization to erythromycin (200mg i.v., age 31±3y, 13 females), or placebo (37±3y, 13 females) in 40 healthy asymptomatic volunteers. Fat-suppressed T2-weighted MR images of the abdomen were acquired on a 1.5T magnet at standard delay times for enterography. Gastric, small, and large intestinal volumes were measured by specialized software. In addition, two radiologists manually measured diameters and percentage distention of jejunal and ileal loops. Treatment effects were evaluated by an ITT analysis based on ANCOVA models.

RESULTS

All subjects tolerated erythromycin. MRI scans of the stomach and intestine were obtained at 62±2 (mean±SEM) and 74±2min respectively after starting oral contrast. Gastric volumes were lower (P<0.0001) after erythromycin (260±49ml) than placebo (688±63ml) but jejunal, ileal, and colonic volumes were not significantly different. However, maximum (76-100%) jejunal distention was more frequently observed (P=0.03) after erythromycin (8/20 subjects [40%]) than placebo (2/20 subjects [10%]). The diameter of a representative ileal loop was greater (P=0.001) after erythromycin (18.8±4.3mm) than placebo (17.3±2.8mm) infusion.

CONCLUSIONS

After ingestion of oral contrast, erythromycin accelerated gastric emptying but effects on small intestinal dimensions were variable. In balance, erythromycin did not substantially enhance small intestinal distention during enterography using current standard delay times.