Graeme C. McKinnon

Towards active guidewire visualization in interventional magnetic resonance imaging

Improving the visibility of interventional devices is of paramount importance if MRI-guided fluoroscopy is to become a reality. Passive visualization is problematic in that the susceptibility-induced artifacts are material- and orientation-dependent. Here a concept is presented for making interventional devices visible. It involves fitting a device with a straight-wire antenna. As the sensitivity of such an antenna is highest for signal sources in the immediate neighborhood, using the antenna for reception gives an outline image. In this manner a guidewire or other interventional device could be made MRI-visible. The image appearance of a straight-wire antenna depends on the orientation of the device with respect to the main magnetic field and imaging plane. This phenomena is discussed theoretically and documented with MR images.

Ultra-high-speed MR imaging

Conventional magnetic resonance imaging (MRI) has been shown to provide excellent morphological images of the body organs, particularly structures undergoing little physiologic motion. Nevertheless, the clinical usefulness of MRI has been hampered by long acquisition times, high cost of scanning because of limited patient throughput, and image artifacts due to patient motion. With recent technical developments, several ultrafast scanning techniques capable of acquiring images in a breath-hold now find their introduction into clinical use. The system improvements are potentially useful for a vast range of applications hitherto not accessible to MR imaging. Among these are functional brain imaging, realtime imaging of cardiac motion and perfusion, fast abdominal imaging, improved MR angiography, and potentially real-time monitoring of interventional procedures. Whereas some ultrafast techniques can be performed on conventional scanners, echo-planar imaging, the fastest currently available data acquisition strategy, requires specially designed hardware. This article provides on overview of the technical advances in the ultrafast MRI and discusses potential applications and the possible future impact on body scanning.

Advances in vascular echoplanar imaging

Magnetic resonance (MR) angiography is emerging as an increasingly useful tool for the noninvasive evaluation of the cardiovascular system. Ultrafast echoplanar imaging (EPI) data acquisition strategies are becoming more widely available. When applied to MR angiography, the associated reduction in data collection time allows breathheld acquisitions of entire vascular territories, as well as the exploitation of short-lived flow-enhancing measures. This paper describes technical aspects of EPI, its implementation into vascular imaging sequences, and associated artifacts. This is followed by a discussion of potential EPI MRA applications in the carotid and renal arteries, the portal venous system, and arteries and veins of the lower leg.

Evaluation of ultrafast phase-contrast imaging in the thoracic aorta

AbstractPurpose: Two ultrafast phase-contrast (PC) data acquisition strategies, multishot echo-planar imaging (EPI)-PC and segmentedk-space fast gradient-echo PC (FASTCARD-PC) were evaluated with regard to their measurement accuracy. Materials and Method: Flow measurements of the ascending and descending aorta were acquired in 10 healthy volunteers with an electrocardiogram (ECG)-triggered eight-shot EPI-PC sequence (TR/TE/flip 16/7.4/45°, 32-ms flow-phase interval, 2×2 mm in plane resolution), and FASTCARD-PC (six it-lines per band, TR/TE/flip 11/6.1/45°, 32-ms flow-phase interval, 2 × 1 mm in plane resolution). These were compared to flow-volume data acquired with conventional cine-PC (TR/TE/flip 24/7/45°, 48-ms flow-phase interval, 2 × 1 mm in plane resolution). Using cine-PC as a gold standard, the measurement accuracy of FASTCARD-PC and EPI-PC were determined. Results: Both EPI-PC and FASTCARD-PC significantly reduced data acquisition times compared to cine PC. EPI-PC flow measurements correlated well with aortic cine-PC flow-volume determinations (r=0.98). Reflecting poorer temporal resolution, FASTCARD-PC measurements were less accurate (p<0.05), evidenced by poor correlation with cine-PC data (r=0.62). Conclusion: Ultrafast PC measurements are possible. In contrast to the segmentedk-space PC technique, which is limited in temporal resolution, multishot EPI-PC provides high measurement accuracy in pulsatile vessels while keeping the image acquisition interval short enough for a comfortable breath-hold.

3d phase contrast epi mr angiography of the carotid arteries

Objective Our goal was to compare 3D phase contrast (PC) echo planar imaging (EPI) MRA of the carotid and vertebral arteries with conventional 3D PC in volunteers and patients. Materials and Methods The carotid arteries of 12 volunteers were imaged with conventional and EPI 3D PC sequences. The visibility for each of seven carotid and four vertebral segments was qualitatively assessed. Signal intensity and homogeneity determinations were performed on the source images. Three patients with known carotid artery disease were also imaged with the same protocol. Results EPI reduced 3D PC data acquisition time from 459 to 32 s (factor of 15). Both techniques permitted full assessment of the common carotid artery, the bifurcation, as well as the proximal internal carotid artery (ICA), external carotid artery (ECA), and vertebral arteries. Visualization of the distal ICA/ECA and vertebral arteries was inferior with EPI compared with the conventional acquisition. In all patients, lesions as established by X-ray angiography were seen to equal advantage with both techniques. Conclusion EPI 3D PC MRA renders diagnostic images of the proximal carotid system. The considerable reduction in data acquisition time must be weighed against poorer image quality.