The value of cosθ varies with the angle from 0 to 1. Table 11.1 shows the relationship between the angle of the Doppler beam (θ) and the value of cosθ. Very small signals are produced as the Doppler beam angle approaches a 90° angle. The smaller the angle between the Doppler beam and blood vessel, the larger the Doppler shifted signal. When the Doppler beam is pointing towards the direction of blood flow a positive Doppler shifted signal is observed, but once the Doppler beam is pointed away from the direction of blood flow a negative Doppler shifted signal is seen. 11.4 Graphically demonstrating the relationship between the Doppler shifted frequency with respect to the angle of the insonating Doppler beam Therefore highly sensitive and sophisticated hardware and processing software is required to ensure that these signals can be detected and processed.įig. The amplitude of the backscattered signal from blood can be smaller by a factor of between 1. It is important to appreciate that the amplitude of the backscattered echoes from blood is much weaker than those from soft tissue and organ interfaces which are used to build up our B-mode anatomical images. Therefore F r = F t and F d = F r − F t = 0, resulting in no Doppler shifted signals. When there is no flow or movement detected then the transmitted frequency (F t) is equal to the received frequency (F r). This time the Doppler shifted frequencies (F r − F t) produces a negative Doppler shifted signal. In this arrangement blood flow moving away from the transducer produces received signals (F r) which have a lower frequency than the transmitted beam (F t). The Doppler shifted signal (F d) can be calculated by subtracting F t from F r and produces a positive Doppler shifted signal.Ĭonversely, Figure 11.3b illustrates blood flow which is moving away from the Doppler beam and the transducer. In this arrangement blood flow moving towards the transducer produces received signals (F r) which have a higher frequency than the transmitted beam (F t). In Figure 11.3a the relative direction of the blood flow with respect to the Doppler beam is towards the transducer. Figure 11.3 illustrates the change in the received backscattered signals and the resulting Doppler shifts for blood moving towards and away from the transducer. 11.3 Demonstrating the resulting Doppler shifted signals for a) blood flow moving towards the transducer b) blood flow moving away from the transducerīlood flow moving towards the transducer produces positive Doppler shifted signals and conversely blood flow moving away from the transducer produces negative Doppler shifted signals. Please allow approximately 30 minutes for each appointment although certain examinations such as morphology (obstetrics) or vascular examinations such as leg arteries or varicose veins require 45 minute appointments.Fig. How long does an examination typically take? Do not empty your bladder prior to the exam. You will need to drink up to one litre of water 60 minutes prior to your scan. What is the preparation for a pelvic/renal ultrasound? Why have I been asked to drink a litre of water for my pelvic/renal ultrasound?Ī comfortably full bladder is required to provide a uniform acoustic window for an optimal assessment of the pelvic structures. Our receptionist will inform you at the time of booking of any preparations that may be required. Failure to fast adequately in this case will result in a contracted gallbladder and subsequently result in a suboptimal examination. For example, you will be requested to fast for a minimum of 6 hours prior to an abdominal examination to allow the gallbladder to be adequately visualised for the assessment of certain pathology such as stones or polyps. Arriving with adequate preparationĬertain ultrasound examinations require appropriate preparation prior to your appointment to ensure optimal assessment of various anatomic structures. It can assist doctors to assess the blood flow through arteries and veins. This is a specialised ultrasound examination studying blood flow within the body. Its applications are varied and multiple, ranging from general abdominal scans, to vascular, musculoskeletal, gynaecological and obstetric examinations. It is a readily accessible, painless tool that can be used to detect many abnormalities without using ionizing radiation. Ultrasound is a medical diagnostic modality which utilises soundwaves to acquire information.
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