Michael O. Zenge1, Justin Ream2, Ankur Doshi2, Mary Bruno2, Christopher Stroehlein3, Peter Speier3, Hersh Chandarana2, and Harald H. Quick4,5
1Siemens Healthcare, Malvern, PA, United States, 2Radiology, NYU Langone Medical Center, New York, NY, United States, 3Siemens Healthcare, Erlangen, Germany, 4Erwin L. Hahn Institute for MR Imaging, University Duisburg-Essen, Essen, Germany, 5High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany
Synopsis
Although unenhanced MR angiography eliminates the need for contrast injection, it would use ECG triggering which adds extra time and complexity to patient preparation. In the current work, time-of-flight (TOF) unlimited introduces self-gated rapid, radial MR imaging with continuous table movement for seamless coverage of the peripheral vasculature. In-vivo experiments in five healthy volunteers and two patients were performed. Initial results are more than promising and TOF unlimited demonstrates similar image quality compared to conventional ECG triggered TOF despite sevenfold accelerated data acquisition. Thus, novel TOF unlimited may be a time efficient screening tool for peripheral arterial disease.Purpose
Computed tomography angiography (CTA) is routinely used to
assess peripheral arterial disease. Recent innovations have resulted in lower
radiation and reduced contrast dose [1]. Nonetheless, the exposure to ionizing
radiation and iodinated contrast remains a concern and has motivated magnetic
resonance angiography (MRA) alternatives. Although unenhanced MRA [2] completely
eliminates the need for contrast injection, the majority of such methods
require ECG triggering which adds extra time and complexity to the patient
preparation. In the current work, conventional time-of-flight (TOF) imaging was
combined with radial data acquisition and continuous table movement [3,4] for
rapid, seamless coverage of the peripheral vasculature. Real-time imaging with
a high frame-rate enables reconstruction in the arterial phase of opacification.
The new method was compared to an ECG triggered TOF reference protocol in five
healthy volunteers. The clinical utility was demonstrated with initial
experience in two patients.
Methods
Novel time-of-flight unlimited (TOF_UNLTD) combines rapid, radial 2D
gradient echo imaging, spatially selective saturation of venous blood and
continuous table movement. In the current prototype, the table speed was
adjusted to complete one slice increment after ten repetitions. Thus, arterial
phase inflow will be covered at least once in each subset of ten images during
a data acquisition window of 1100 ms. Non-Cartesian image reconstruction was performed
online using table based gridding [5]. Thereafter, arterial MRA images were generated
from the input stream of images with a sliding maximum intensity projection (MIP).
Table 1 documents the MR imaging protocols of TOF_UNLTD versus conventional
ECG triggered peripheral TOF (ECG_TOF). In-vivo experiments in 5 healthy
volunteers (3 female, 2 male, 23 – 32 years) were performed under a local IRB approved
study. Imaging was performed on a 3.0T MR system (MAGNETOM Skyra, Siemens
Healthcare, Erlangen, Germany) using a combination of one body array surface
coil, spine array coils and a dedicated peripheral MRA coil. The image quality
was rated by two board-certified radiologists in multiple vessel sections on a four
point scale. The clinical utility of TOF_UNLTD was demonstrated in two patients
with known peripheral arterial disease.
Results
TOF unlimited (Figure 1) was performed successfully in all volunteers and patients and demonstrated a sevenfold reduction in total acquisition time compared
to conventional ECG triggered TOF (6:35 min vs. 34:00 – 44:40 min). In addition,
the small continuous slice increment in combination with a large flip angle
resulted in visually superior background suppression over the ECG_TOF reference (Figure 1). Despite the fact that the data acquisition was highly undersampled
in the case of TOF_UNLTD only minor streaking artifacts with no impact on the
diagnostic accuracy were observed. Although the overall image quality was rated
qualitatively the same for both methods (Table 2), the ECG triggered
reference images showed higher relative signal intensity (Table 3). Finally, TOF_UNLTD
proved to demonstrate similar diseased segments as the contrast enhanced MRA in
two patients (Figure 2).
Discussion
The results demonstrate that the image quality of TOF unlimited compared
similar to conventional ECG triggered TOF despite nearly six to sevenfold
shorter acquisition time. Although the image quality was rated excellent for
both methods, the following modifications ought to be considered: An implementation of iterative SENSE [6] could reduce streaking
artifacts and, by virtue of the anticipated improvement of SNR would enhance
the vessel conspicuity. Another technical advancement of TOF_UNLTD which could
improve the relative signal intensity might be extending the temporal footprint
of the input data via view sharing to better utilize the available arterial
signal. Furthermore, novel image processing such as low-rank and sparse matrix
decomposition [7] has great potential to be used as an alternative self-gating
approach which might help to avoid residual blur caused by the featured sliding
MIP approach. Besides a number of potential improvements, it seems worth noting
that TOF_UNLTD already proved to be twice as fast as a competitive self-gated
unenhanced MRA method [8] which was published not long ago. Therefore, a systematic comparison of TOF_UNLTD not solely to contrast enhanced MRA is highly motivated.
Conclusion
Time-of-flight unlimited presents a novel rapid unenhanced method for
peripheral MRA with continuous table movement which was successfully applied to
volunteers and patients. The initial success suggests great clinical potential
because the method is likewise fast and easy to use. Future research will require
the clinical evaluation of the proposed method in a large cohort of patients. But unlike
CTA and contrast enhanced MRA, TOF unlimited may be a time efficient screening
tool for peripheral arterial disease unencumbered by the threat of latent renal
insufficiency.
Acknowledgements
No acknowledgement found.References
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