A reduced field-of-view method to increase temporal resolution or reduce scan time in cine MRI.

In some dynamic applications of MRI, only a part of the field-of-view (FOV) actually undergoes dynamic changes. A class of methods, called reduced-FOV (rFOV) methods, convert the knowledge that some part of the FOV is static or not very dynamic into an increase in temporal resolution for the dynamic part, or into a reduction in the scan time. Although cardiac imaging is an important example of an imaging situation where changes are concentrated in a fraction of the FOV, the rFOV methods developed up to now are not compatible with one of the most common cardiac sequences, the so-called retrospective cine method. The present work is a rFOV method designed to be compatible with cine imaging. An increase by a factor n in temporal resolution or a decrease by n in scan time is obtained in the case where only one nth of the FOV is dynamic (the rest being considered static). Results are presented for both Cartesian and spiral imaging.

Temporal resolution improvement in dynamic imaging.

In some dynamic imaging applications, only a fraction, 1/n, of the field of view (FOV) may show considerable change during the motion cycle. A method is presented that improves the temporal resolution for a dynamic region by a factor, n, while maintaining spatial resolution at a cost of square root of n in signal-to-noise ratio (SNR). Temporal resolution is improved, or alternatively, total imaging time is reduced by reducing the number of phase encodes acquired for each temporal frame by 1/n. To eliminate aliasing, a representation of the signal from the static outer portion of the FOV is constructed using all the raw data. The k-space data derived from this representation is subtracted from the original data sets, and the differences correspond to the dynamic portion of the FOV. Improved resolution results are presented in phantom studies, and in vivo phase contrast quantitative flow imaging.

Simultaneous temporal resolution of cardiac and respiratory motion in MR imaging.

PURPOSE: To evaluate a magnetic resonance (MR) imaging method that allows for simultaneous resolution of both the cardiac and respiratory cycles. MATERIALS AND METHODS: Conventional and phase-contrast cine sequences were modified to provide additional resolution of the respiratory cycle. Data were collected in 11 healthy volunteers during MR imaging of the heart and portal vein. The imaging time was increased over that of a conventional cine acquisition by a factor equal to the number of frames in the respiratory cycle. Data were compared with those from comparable sequences in which only one motion cycle was resolved. RESULTS: In the heart, motion due to cardiac dynamics was separated from respiration-induced excursions. The extent of motion could be measured, and artifacts were minimized. Changes in flow rate as a function of both motion cycles were resolved and quantified in both the portal vein and superior vena cava. CONCLUSION: This method allows for simultaneous resolution of cardiac and respiratory motion cycles and helps provide a more physiologic view of the effects of cardiac and respiratory variations.

Time-resolved MR imaging by automatic data segmentation.

A method for time-resolved imaging that provides a flexible trade-off between imaging time and temporal resolution is presented. It is based on a view order selection technique that automatically segments the acquired raw data into appropriate temporal frames. When used with cardiac monitoring and phase-contrast imaging, data similar to that obtained with a conventional gated phase-contrast sequence are acquired rapidly. For many applications, the temporal resolution can be reduced enough to permit imaging within a breath-hold interval, while still allowing accurate time-averaged flow quantitation. This is a general technique that can be implemented within a variety of pulse sequences and can resolve other motion cycles, including the respiratory cycle.

Renal artery blood flow: quantitation with phase-contrast MR imaging with and without breath holding.

PURPOSE: To compare the accuracy of 16-frame cine phase-contrast (PC) magnetic resonance (MR) imaging with those of two breath-hold PC techniques in the measurement of renal artery blood flow. MATERIALS AND METHODS: In vitro flow measurements were performed in a segment of harvested human artery embedded in gel. For the cine PC acquisition, respiratory motion was simulated. In eight subjects with recently obtained para-amino-hippurate-clearance renal blood flow data, renal artery flow measurements were subsequently performed with two breath-hold imaging techniques and with cine PC imaging during shallow respiration. RESULTS: Breath-hold sequences were significantly more accurate than conventional cine PC sequences both in vitro (P < .005) and in vivo (P < .05). Cine PC imaging tended to overestimate flow (in vivo mean, 24.47% +/- 9.94), reflecting artifactual enlargement of the apparent vessel size. CONCLUSION: Reliable blood flow measurements in the renal artery are possible with breath-hold PC MR imaging.

Chronic mesenteric ischemia: evaluation with phase-contrast cine MR imaging.

PURPOSE: To compare superior mesenteric artery (SMA) blood flow in healthy volunteers and patients with stenoses in the fasting state and after food intake by using phase-contrast (PC) cine magnetic resonance (MR) imaging. MATERIALS AND METHODS: Ten healthy subjects, four asymptomatic patients (three with 50% stenosis, one with 70% stenosis), and one symptomatic patient (with 80% stenosis) were studied. All subjects were studied after fasting at least 8 hours and 15, 30, and 45 minutes after ingesting a standard meal. RESULTS: In healthy volunteers, SMA blood flow at all postprandial intervals increased significantly compared with that obtained after fasting (P < or = .0005). The percentage change in SMA blood flow 30 minutes after food intake provided the best distinction between the healthy subjects, the asymptomatic patients, and the symptomatic patient. CONCLUSION: Cine PC MR imaging is an effective, noninvasive technique for measuring SMA blood flow.