## 1 Introduction  
Pulse wave velocity (PWV) refers to the speed at which a pulse wave propagates through a blood vessel. An elevated PWV is indicative of increased arterial stiffness, which leads to reduced arterial elasticity and restricted vascular dilation, resulting in higher pressure within the vessel [1]. Owing to these characteristics, PWV serves as a crucial physiological marker for assessing arterial stiffness, enabling early diagnosis of cerebrovascular diseases, such as cerebral aging and infarction [2, 3, 4, 5, 6, 7]. Representative noninvasive methods for measuring PWV in blood vessels include phase-contrast magnetic resonance imaging (MRI), arterial tonometry, and Doppler ultrasound [8, 9]. The measurement of PWV using tonometry involves placing pressure sensorsat two points, typically along the carotid and femoral arteries, to record arterial pressure waveforms. The pulse wave transit time between two measuring sites is measured, and PWV is calculated by dividing the distance between these sites by the transit time [10; 11]. Although tonometry has the advantage of providing direct pressure waveforms, its accuracy can vary depending on sensor placement and the patients' vascular condition, thereby affecting measurement consistency [12]. Additionally, when calculating the distance between two measurement sites, this method uses a certain ratio based on the patient's height rather than using the actual length of the blood vessels, which can reduce the accuracy of the distance [13]. Another method for measuring PWV involves the use of Doppler ultrasound. In this technique, an ultrasound probe is placed at the vessel of interest, such as the carotid artery, and the blood pressure and diameter information at the site are obtained to calculate the stiffness index and PWV [14; 15]. Doppler ultrasound is efficiently utilized in clinical settings because it does not require special preparation or recovery time and is a simple and quick measurement technique [16; 17]. However, the accuracy of Doppler measurement can vary based on the precise placement and angle of the probes, making the results highly dependent on the operator's skill [9; 18; 19; 20; 21]. Additionally, because of the low ultrasound penetration rate, this method may be limited in obtaining information from deeper blood vessels, such as cerebral vessels, as it can be affected by factors, such as skin thickness, tissue density, and bone [22; 23]. The limitations of PWV measurement using tonometry and Doppler ultrasound can be overcome by the features of MRI, which offers high resolution and contrast among various tissues, as well as the ability to quantitatively assess deep blood vessels [24; 25]. Using MRI sequences with a high temporal resolution, blood flow signal waveforms can be obtained at two arterial locations, and the pulse wave transit time can be calculated by measuring the peak difference between two blood flow signal waveforms [8]. PWV is then calculated by dividing the distance between two imaging points by the transit time. Simultaneous multislice imaging (SMS) with a multiband radio frequency pulse, which excites multiple slices simultaneously, allows for the simultaneous acquisition of images from two different blood vessels, eliminating the need to synchronize the blood flow timing in the two vessels for pulse wave transit time calculation [26]. Combining the keyhole technique with the SMS method (SMS-K) enables rapid scanning, allowing for the evaluation of real-time changes in PWV, thereby measuring dynamic vascular elasticity over time [27]. In a previous study, the SMS-K technique was proposed to measure cerebrovascular stiffness by generating vascular waveforms and calculating PWV in the internal and common carotid arteries (ICA and CCA), thus confirming the basic research findings and clinical utility of the SMS-K method [27]. In the SMS-K technique, undersampling of the k-space with parallel encoding during image reconstruction can result in a vertical projection effect.