Max CAPR: high-resolution 3D contrast-enhanced MR angiography with acquisition times under 5 seconds

Abstract

High temporal and spatial resolution is desired in imaging of vascular abnormalities having short arterial-to-venous transit times. Methods that exploit temporal correlation to reduce the observed frame time demonstrate temporal blurring, obfuscating bolus dynamics. Previously, a Cartesian acquisition with projection reconstruction-like (CAPR) sampling method has been demonstrated for three-dimensional contrast-enhanced angiographic imaging of the lower legs using two-dimensional sensitivity-encoding acceleration and partial Fourier acceleration, providing 1mm isotropic resolution of the calves, with 4.9-sec frame time and 17.6-sec temporal footprint. In this work, the CAPR acquisition is further undersampled to provide a net acceleration approaching 40 by eliminating all view sharing. The tradeoff of frame time and temporal footprint in view sharing is presented and characterized in phantom experiments. It is shown that the resultant 4.9-sec acquisition time, three-dimensional images sets have sufficient spatial and temporal resolution to clearly portray arterial and venous phases of contrast passage. It is further hypothesized that these short temporal footprint sequences provide diagnostic quality images. This is tested and shown in a series of nine contrast-enhanced MR angiography patient studies performed with the new method.

FIG. 1

Comparison of the sampling patterns of the k_Y-k_Z phase encoding plane for the Cartesian CAPR technique (a) and the non-view-shared Max CAPR technique (b). c: Temporal play-out of phase-encoding measurements used for both CAPR and Max CAPR. d,e: Selection of the data from (c) for formation of images 1 and 2 for the CAPR sequence. f,g: Selection of data for formation of images 1 and 2 for the Max CAPR sequence. The same sampling of the central orange region is used for corresponding CAPR and Max CAPR images.

FIG. 2

Plot of temporal footprint versus image update time for fixed Y × Z sampling of 320 × 132. Curves are shown for SENSE accelerations R = 4 and 8. The parameter N1, N2, etc. corresponds to increased degrees of view sharing, providing a reduced update time but increased temporal footprint. For Max CAPR (red line) no view sharing is performed, and so the update time matches the temporal footprint. For much of this work, N4 Max CAPR was studied.

FIG. 3

Axial slices of three tubes of diameter 3, 5, and 7mm acquired with increasing acceleration. a: Image from fully sampled k_Y-k_Z-space. Acquisition time was 193.3 sec. b,d,f,h: Reference CAPR reconstructions formed using 2D HD and the SENSE accelerations (R) shown. For each case, the acquisition time and the net acceleration R_net versus (a) are indicated. c,e,g,i: Max CAPR images reconstructed from the same respective data sets as for CAPR but using only one of the four vane sets. For each, the acquisition time and R_net are given.

FIG. 4

Enlargements of the leading and trailing edges of the moving phantom, each from distinct timeframes, for (a) CAPR Delay 0, (b) CAPR Delay 3, and (c) Max CAPR reconstructions. For each case, the colored blocks indicate which region of k-space is being sampled at the instant the bolus edge is at the position within the block. Note that CAPR Delay 0 has substantial persistence artifact ((a), arrow), CAPR Delay 3 has substantial anticipation artifact ((b), arrow), but Max CAPR (c) has negligible artifact in advance of and trailing the bolus edges.

FIG. 5

Comparison of CAPR (17.6-sec acquisition time) and Max CAPR (4.9-sec acquisition time). Full coronal MIP images of (a) CAPR and (b) Max CAPR results showing the leading edge of contrast material as it enters the proximal portion of the lower legs over the full FOV. Enlargements of the right leg of CAPR (c) and Max CAPR (d) portray the bifurcation of the popliteal artery. Note the increased noise level within the dashed box for CAPR (c) versus Max CAPR (d). Enlargements of the left leg from CAPR (e) and Max CAPR (f) illustrate improved sharpness in portraying the leading edge of the contrast bolus for Max CAPR ((f), arrows).

FIG. 6

Plots of the evaluation scores of the nine patient studies for each of the six categories defined in Table 2. For each category, results are shown for the two reviewers, a total of 18 scores for Max CAPR and 18 scores for CAPR.

FIG. 7

Comparison of CAPR (17.6-sec acquisition time) versus Max CAPR (4.9-sec acquisition time) from a patient study. Coronal MIP of full FOV of CAPR (a) and Max CAPR (b) for the frame when contrast material nominally fills the arteries within the calves. c: Max CAPR result one timeframe (4.9 sec) later than (b). Comparison of (d) CAPR and (e) Max CAPR subvolumes of the left popliteal bifurcation (dashed boxes in (a) and (b)) shown at an oblique angle to better portray the two stenotic areas, one at the origin of the anterior popliteal artery (long arrow, (d) and (e)), the second several centimeters distal (short arrow, (d) and (e)). Although the Max CAPR result is slightly blurred versus CAPR, it still portrays the pathology well. Axial images of CAPR (f) and Max CAPR (g) taken from the level of the dashed lines from the respective subvolumes of (d) and (e) show the dark vessel lumens. Note the increased noise level in the background of CAPR. See also Supplementary Videos V1 and V2.

FIG. 8

Illustration from a patient study of the spatial and temporal resolution of Max CAPR (4.9-sec acquisition time). a–c: Coronal MIPs of consecutive 4.9-sec timeframes in a patient having a lesion at the origin of the right popliteal bifurcation (arrow, (a)) and filling via collateral vessels of the vasculature of the left leg. d–f: Enlargements from the region of the left leg (dashed boxes, (a–c)), allowing delineation of the filling patterns. d: Filling of the native distal left posterior tibial artery (long arrow, (d)) is done via a medial collateral vessel (short arrows, (d)). Second collateral vessel (arrowheads, (d)) spontaneously anastomoses to a distal native peroneal artery, with an early hint of retrograde flow (curved arrow, (d)). e: Subsequent frame shows further retrograde flow along peroneal artery (curved arrow, (e)), as well as a more proximal anastomosis in the peroneal artery (short arrow, (e)) from a third collateral vessel (arrowheads, (e)). f: Next frame shows further enhancement, including filling of native left anterior tibial artery (long arrow, (f)). See also Supplementary Videos V3 and V4.