Peripheral arterial disease (PAD), a type of systemic arterial disease affecting arteries other than the coronary arteries and aorta, is caused by atherosclerosis [1]. PAD deteriorates the quality of life, presenting with symptoms such as intermittent claudication, pain at rest, and gangrene. Risk factors for PAD include aging, smoking, diabetes, hypertension, dyslipidemia, and renal insufficiency [2,3]. The prevalence of PAD has increased worldwide with an increase in the aging global population and incidence of chronic non-communicable diseases. The anatomic location and degree of stenosis in PAD can be assessed using computed tomography angiography (CTA) and magnetic resonance angiography (MRA), each offering distinct advantages and limitations. CTA provides high spatial and temporal resolutions, making it a commonly used modality in clinical practice [4]. However, visualizing the lumen of small, calcified arteries below the knee is challenging when using ionizing radiation-based imaging [5]. Additionally, iodine-based contrast agents are contraindicated in patients with renal insufficiency, which is a cause of PAD. Contrast enhanced-MRA (CE-MRA) is also contraindicated in these patients because gadolinium-based contrast agents can cause nephrogenic systemic fibrosis [6].

Owing to these limitations, non-CE-MRA (NCE-MRA) is particularly valuable for patients with renal insufficiency. In addition to conventional NCE-MRA techniques such as time-of-flight (TOF) [7] and fresh blood imaging (FBI) [8], other NCE-MRA techniques exploit blood flow characteristics by applying a flow-sensitized dephasing (FSD) gradient [9] or an acceleration selective-motion sensitized gradient (AS-MSG). By modifying the AS-MSG strength, blood flow dephasing can be adjusted to enhance arterial visualization. This technique is known as acceleration-dependent FSD or enhanced acceleration-selective arterial spin labeling (eAccASL) [10,11]. Priest et al. [12] showed that acceleration-dependent FSD with peripheral pulse triggering improves the separation of arteries and veins, enabling selective visualization of the arteries below the knee. However, some patients with PAD have different blood flow profiles between the legs, which makes determining the optimal trigger delay time for acceleration-dependent FSD more difficult when using electrocardiography (ECG) or peripheral pulse unit (PPU) [13]. This results in poor arterial visualization in one leg or requires separate acquisitions for each leg. Furthermore, ECG triggering is susceptible to issues such as gradient switching and arrhythmia. Most NCE-MRA techniques have same problem because of the requirement for ECG or PPU triggering. Therefore, several reports have recently been published on lower-extremity MRA using non-triggered, non-contrast enhanced eAccASL [14,15]. The eAccASL acquisition with adjusted AS-MSG strength allows observing differences in pulsatility between arterial and venous blood flow without ECG triggering. In the lower extremities, non-triggered eAccASL with turbo spin-echo (TSE) is superior to non-triggered eAccASL with turbo field-echo (TFE) [15]. In this study, we selected the TFE sequence to compare non-triggered eAccASL with ECG-triggered eAccASL because ECG-triggered eAccASL with TSE requires different cardiac phases for control and label acquisitions. eAccASL did not allow control and labeling acquisitions to be performed at different cardiac phases because it is a work in progress. In this study, we aimed to investigate the appropriate strength of the AS-MSG for non-triggered, non-contrast enhanced eAccASL compared with ECG-triggered eAccASL.