Magnetic resonance angiography (MRA) of the extremities can help in the diagnosis of conditions such as peripheral vascular occlusion, peripheral arterial disease and Raynaud’s disease. The main challenges for this application are the inherently slow blood flow and large volumetric coverage requirements in the lower extremities.
To date, there have been two main groups of MRA techniques for the extremities. Contrast-enhanced methods have been successfully used in the lower extremities (1
); however, the bolus timing requirements limit the spatial resolution and signal-to-noise ratio (SNR). Furthermore, administration of gadolinium-based contrast agents introduces the risk of nephrogenic systemic fibrosis in patients with renal disease (5
). Therefore, there has been renewed interest in non-contrast-enhanced MRA methods. Most of these techniques can effectively generate the desired blood-to-background contrast by relying on flow. However, the performance of flow-based techniques such as phase-contrast (6
) or time-of-flight angiography (8
) can be limited by reduced blood flow rate in the extremities, particularly with severe atherosclerotic disease. There are other flow-dependent techniques that can cope with slow flow more successfully such as fresh-blood imaging (FBI) (10
). FBI subtracts diastolic- and systolic-triggered fast spin-echo acquisitions to produce high-resolution angiograms with reliable background suppression, but improper timing of the trigger delays can lead to blood signal loss in the subtraction images.
While most MRA techniques rely on contrast agents and/or flow to generate contrast, flow-independent angiography (FIA) (12
) exploits differences in T1 and T2. This allows FIA to produce vessel contrast even in cases of slow flow. In early FIA work, magnetization-preparation schemes were combined with fast 3D imaging to generate contrast; however, the SNR efficiency was limited (13
). Recently, FIA angiograms have been produced with magnetization-prepared balanced steady-state free precession (bSSFP) sequences (14
) and centric phase-encode ordering to overcome this limitation (16
Although bSSFP yields high SNR within short scan times, bright fat signal often obscures visualization of the underlying vasculature. In this study, we examined three different methods for reducing the fat signal in FIA angiograms: phase-sensitive (PS) SSFP (17
), alternating repetition time (ATR) SSFP (18
), and a new double-acquisition ATR-SSFP method (19
). PS-SSFP is fast and efficient, but partial volume averaging can cause loss of blood signal in the vicinity of bone marrow and subcutaneous fat tissue. ATR-SSFP can instead be used to create a stop-band around the fat resonance (18
) and reduce partial volume artifacts with little increase in scan time. However, the level of suppression is more sensitive to field inhomogeneity than PS-SSFP. In these cases, a new double-acquisition ATR-SSFP method can be employed to improve the stop-band at the expense of lengthened scan time (19
We can produce FIA angiograms of the extremities with detailed depiction of the vasculature within several minutes, without the need for an intravenous contrast agent. The desired vessel contrast is generated by coupling magnetization-prepared three-dimensional Fourier transform (3DFT) bSSFP acquisitions with adequate fat suppression techniques. In cases where partial volume effects are not dominant (e.g., resolution < 0.7 mm3 in the extremities), PS-SSFP might be preferred to decrease the sensitivity to field inhomogeneity. However, ATR-based techniques generate more reliable contrast in the presence of considerable partial voluming.