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.

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.

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).

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.

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.