Nonenhanced methods for lower‐extremity MRA: A phantom study examining the effects of stenosis and pathologic flow waveforms at 1.5T

Peripheral arterial disease (PAD) has increased prevalence in older populations, smokers, and diabetics, and is associated with systemic atherosclerosis and cardiovascular morbidity (1–4). The ankle-brachial index (ABI) is a useful screening test, but is less reliable in diabetic patients who exhibit arterial calcification (5,6). Moreover, it is generally unable to determine the precise location and severity of vascular stenosis, so that an additional imaging test is needed prior to revascularization. Additional imaging tests include contrast-enhanced magnetic resonance angiography (CE-MRA), which has proven to be accurate and avoids exposure to ionizing radiation and potentially nephrotoxic iodinated contrast agents (7). Renal insufficiency is common in patients with PAD (8,9). This poses a problem for CE-MRA since it requires injection of a gadolinium-based contrast agent (Gd), which has been linked to nephrogenic systemic fibrosis (NSF) in patients with renal failure (10,11). The continued clinical demand for MRA in PAD evaluation requires scanning techniques that are safe for renally impaired patients. This has led to investigation into nonenhanced MRA techniques that do not require the use of contrast agents (12). Several nonenhanced MRA techniques for evaluating the lower extremities have been proposed. Time of flight (TOF) is a method that relies on inflow enhancement (13). However, TOF is slow and requires rapid flow for optimal performance. Electrocardiogram (ECG)-gated 3D partial Fourier fast spin-echo (also termed “fresh blood imaging”) (FBI) is a technique that subtracts a “bright-blood” diastolic acquisition from a “black-blood” systolic acquisition, creating an angiogram with high vessel contrast and little background signal (14). However, FBI requires preparatory scans to define the acquisition windows and optimal spoiler gradient strengths, which can lead to operator variability and prolong study time. Ungated ghost MRA (Ghost) is a technique that uses ghost artifacts in the slice-encoding direction to produce an angiogram (15). Ghost does not require ECG gating, but is limited in that it can only produce a 2D angiogram. A more recent development is quiescent-interval single-shot (QISS) (16), which consists of a balanced steady-state free precession (bSSFP) readout applied during slow diastolic flow. The method uses presaturation to suppress background tissue and venous signal while arterial signal is enhanced by inflow during a quiescent interval that overlaps the period of fast arterial flow. Little is known about the performance characteristics of these nonenhanced MRA techniques. Given that a study of all techniques mentioned above is not practical in human subjects due to parametric and scan time limitations, we sought to determine the performance characteristics of the TOF, FBI, Ghost, and QISS MRA methods by measuring signal versus velocity using a pulsatile flow phantom. Triphasic and monophasic flow profiles, blood-mimicking fluid, and a tapered stenosis were used to simulate normal and diseased conditions found in vivo.