Vascular masking for improved unfolding in 2D SENSE-accelerated 3D contrast-enhanced MR angiography

Abstract

Purpose: To describe and evaluate the method we refer to as "vascular masking" for improving signal-to-noise ratio (SNR) retention in sensitivity encoding (SENSE)-accelerated contrast-enhanced magnetic resonance angiography (CE-MRA).

Materials and methods: Vascular masking is a technique that restricts the SENSE unfolding of an accelerated subtraction angiogram to the voxels within the field of view known to have enhancing signal. This is a more restricted voxel set than that identified with conventional masking, which excludes only voxels in the air around the object. Thus, improved retention of SNR is expected. Evaluation was done in phantom and in vivo studies by comparing SNR and the g-factor in results reconstructed using vascular versus conventional masking. A radiological evaluation was also performed comparing conventional and vascular masking in R = 8 accelerated CE-MRA studies of the thighs (n = 21) and calves (n = 13).

Results: Images reconstructed with vascular masking showed a significant reduction in g-factor and improved retention of SNR versus those reconstructed with conventional masking. In the radiological evaluation, vascular masking consistently provided reduced background noise, improved luminal signal smoothness, and better small vessel conspicuity.

Conclusion: Vascular masking provides improved SNR retention and improved depiction of the vasculature in accelerated, subtraction 3D CE-MRA of the thighs and calves.

Keywords: SENSE; contrast-enhanced MR angiography (CE-MRA); masking.

Figure 1

Diagrams of a conventionally masked (a) and vascular-masked (b) abdomen cross-section with enhanced right and left renal arteries (RRA, LRA), perfused right and left kidneys (RK, LK), and enhanced abdominal aorta (Ao). With 4 × 2 Cartesian SENSE acceleration, a voxel within the aorta is aliased with seven other voxels (gray). Three of the aliased voxels are in air and contain no signal in the unsubtracted case, while in the subtracted case an additional four voxels contain subtracted static tissue. Conventional masking removes the three non-signal-producing aliased voxels in air. In this example, vascular masking removes all seven of the non-signal-producing aliased voxels.

Figure 2

The g-factors calculated from 2¹³ random sensitivity matrices and sorted according to the g-factor from the reconstruction with no mask. Note that in all cases, reducing the size of the sensitivity matrix by masking reduces the g-factor.

Figure 3

Flowchart showing the reconstruction process for SENSE unfolding with conventional masking (a) and vascular masking (b). Under the assumption of a near-ideal subtraction, vascular masking uses the result from the conventionally masked reconstruction to create a more restrictive vascular mask to use in the SENSE unfolding.

Figure 4

A phase-encode plane axial partition of an R_Y × R_Z = 4 × 2 accelerated image of the bovine gelatin phantom shows the difference in masks (a-c), unfolded subtracted images (d-f) and g-factor maps (g-i) for different voxel exclusion masks. Images reconstructed with no mask (a, d, g), the conventional mask (b, e, h), and the vascular mask (c, f, i) are shown.

Figure 5

Relative SNR (a) and mean g-factor (b) in phantom images reconstructed with no mask, the conventional mask, and the vascular mask. The theoretical values in (a) are for g = 1 and $\mathit{\text{SNR}}\alpha \frac{1}{\sqrt{R}}$.

Figure 6

Histograms of the results of the radiological evaluation for the thigh and calf studies. Categories I – III are defined in Table 2. In Categories I and III, the null hypothesis of “no improvement” was rejected in a statistically significant way, showing that images reconstructed with vascular masking have a radiological improvement compared to images reconstructed with conventional masking. In Category II, there was no significant loss of vessels due to masking.

Figure 7

Full-FOV and rotated targeted MIPs of R = 8 2D SENSE-accelerated 3D CE-MRA thigh angiograms reconstructed with the conventional masking technique (a-b) and the new vascular masking technique (d-e). Representative axial slices of the datasets are shown in (c) and (f). The position of the axial slices is shown by the dotted line on the full-FOV MIPs. The image reconstructed with the vascular mask (e) shows improved luminal signal smoothness (arrow heads) and better small vessel conspicuity (arrow) vs. conventional masking (b).

Figure 8

Full-FOV and targeted MIPs from an R=8 2D SENSE-accelerated 3D CE-MRA calf study reconstructed with the new vascular mask (a, c) and the conventional masking technique (b). Note the improved small vessel conspicuity (arrows) in the image reconstructed using the vascular mask.

Figure 9

In vivo relative SNR (a-b) and g-factor (c-d) plots of paired measurements from images reconstructed with the conventional mask and the vascular mask. Thigh results (n = 21) are shown in (a, c), and calf results (n = 13) are shown in (b, d).

Figure 10

Alias counts per axial slice for the thigh study shown in Figure 7 (a-b) and a calf study (c-d). Note that the vascular masking reduces the alias count per slice greatly and eliminates 7- and 8-fold aliasing.