First Clinical Investigations of New Ultrasound Techniques in Three Patient Groups: Patients with Liver Tumors, Arteriovenous Fistulas, and Arteriosclerotic Femoral Arteries

Abstract

In this PhD project two newer ultrasound techniques are for the first time used for clinical scans of patients with malignant liver tumors (Study I), arteriovenous fistulas for hemodialysis (Study II) and arteriosclerotic femoral arteries (Study III). The same commercial ultrasound scanner was used in all three studies. Study I was a comparative study of B-mode ultrasound images obtained with conventional technique and the experimental technique Synthetic Aperture Sequential Beamforming (SASB). SASB is a datareducing version of the technique synthetic aperture, which has the potential to produce ultrasound images of very high quality with high frame rate. Synthetic aperture is unfortunately very demanding computationally, and is therefore used only in experimental scanners. SASB reduces the data volume by a factor of 64, thereby making it possible to implement the technology on a commercial ultrasound scanner, to perform wireless data transfer and in the future to develop e.g. a wireless ultrasonic transducer. Nineteen patients with either primary liver cancer or liver metastases from colon cancer were ultrasound scanned the day before planned liver resection. Patients were scanned simultaneously with the conventional technique and SASB, and the image quality was subsequently evaluated from a clinical perspective by five radiologists with ultrasound experience. The evaluations showed a slight (statistically insignificant) advantage to SASB, and the study thereby showed that SASB, in spite of the significant data reduction, is suitable for clinical use. In Study II, 20 patients with arteriovenous fistulas for hemodialysis were ultrasound scanned directly on the most superficial and accessible part of the fistula. The vector ultrasound technique Vector Flow Imaging (VFI) was used. VFI can quantitatively estimate the direction and velocity of the blood flow in a vessel, independently of the angle of insonation. Conventional Doppler technique is dependent on an angle of insonation < 60-70° when a quantitative estimation of flow is needed. It is therefore challenging to use on the very superficial arteriovenous fistulas. The fistulas were scanned perpendicular to the vessel, the cross-sectional area was calculated and blood flow velocity measured. The average flow velocity was calculated and multiplied by the cross sectional area, thereby calculating volume flow in the fistula. This was compared with the gold standard for volume flow measurements (ultrasound dilution technique), and was 31 – 35 % lower than the gold standard, but showed a 4 significantly improved standard deviation. The study thus demonstrated a new, direct and intuitive way to measure blood flow in arteriovenous fistulas. Study III was also a flow study using VFI. Eleven patients with arteriosclerotic disease in the superficial femoral artery had an ultrasound scan of the vessel performed just before a planned angiography of the arteries. If turbulent/disturbed flow was identified with VFI, and suspicion of a flow disturbing arteriosclerotic lesion was raised, recordings of the flow were made. The recordings were subsequently analyzed, and for each recording blood flow velocity at the lesion was compared with the flow velocity in a healthy adjacent arterial segment. If the velocity at the lesion was higher than in the healthy segment, it was considered a stenosis. By comparison with the subsequent angiography a strong correlation was found between the calculated velocity ratios and the measured angiographic stenosis degrees. Thus, it was possible to assess stenosis degree quantitatively from the VFI ultrasound scan. Furthermore, it was calculated that a doubling of the flow velocity indicates a stenosis degree of 50 %, and thus a clinically significant stenosis requiring treatment. The study is the first of its kind where a vector ultrasound technique is used to calculate velocity ratios related to arteriosclerotic stenoses, and the obtained results are consistent with previous studies performed with conventional Doppler technique. Use of VFI is more intuitive, and may be used to perform faster and more accurate screening of these patients before they are referred to angiography. The three studies demonstrate the first application of the new ultrasound techniques in selected groups of patients. For all three studies the results are promising, and hopefully the techniques will find their way into everyday clinical practice for the benefit of both patients and healthcare practitioners.