A Speckled Future

Innovations in science, especially in Medicine, usually arise at the interface of classic scientific disciplines. When new technology is used in medicine for diagnosis or interventions on our patients, physicians should try to understand the principles of the technology, the limitations and capabilities. Clinical applications of ultrasound have evolved greatly since the first fuzzy pictures were created in the 1960s.1 Ultrasound is now an indispensable tool in evaluation of patients with all kinds of kidney problems, in evaluation of vascular grafts and fistulas in dialysis patients, and in choosing proper blood vessels for creation of successful arteriovenous fistulas (mapping). Vein and artery mapping by ultrasound is done to choose arteries and veins which allow a fistula to be created and have the potential to allow the fistula to mature. Fistula maturation requires both the vein and artery to outwardly remodel to become much larger than the native vessels. After all, the properly functioning fistula must carry about 10 times as much blood flow as did the native vessels before surgery. The size of blood vessels is fairly easily estimated on standard “B-mode” ultrasound images, although user skill and judgment affect the choice of vessels to image, the appearance of the images, and the measurements on a computer screen. Looking at a somewhat fuzzy ultrasound image of a 2 to 4 mm vein or artery, it is not easy to determine exactly where the is border of intima, blood and media, and therefore, the exact internal diameter is a guess. More importantly, it is not just the size of vessels but their distensibility and compliance that determine whether they will enlarge over time to create a fistula which matures to a size and flow rate to successfully allow access for dialysis. Measuring vessel compliance (distensibility) is even more difficult and imprecise than measuring the size of the vessels. The standard technique for veins is to obtain images of the vessels in a resting arm, apply a tourniquet to enlarge the vein, and then obtain images of the same vessels at full expansion. But this means that measurements from two separate images are needed to determine the expansion. The transducer may not be in exactly the same place on the vein in each measurement. Further, the venous pressure increase which occurs with a tourniquet application is not always the same. Measures of arterial vessel compliance have similar subjectivity and uncertainty in image collection and measurement. In this issue of ASAIO Journal, Funes-Lora et al describe a sophisticated program to analyze standard ultrasound images (in DICOM) to determine the exact location of the intimal border of a blood vessel, the maximum diameter of the inside of the vessel, and the increase in diameter as the vein expands with each heartbeat.1 The method automatically determines speckle points, which are represent points in the image of the intima which arise from a mixture of interference patterns and natural acoustic reflections.2,3 The program then calculates the change in distance between these points during successive heartbeats. The authors performed a clinical trial using ultrasound images of the brachial artery of normal volunteers, and the distensibility of the vessel with each heartbeat was determined by the computer program. The distensibility was also calculated manually by 10 experts in ultrasonography examining the same images. The changes in venous vessel diameters with each heartbeat is really tiny, in the range of 0.1 to 0.2 mm, with each heartbeat. As you might guess, the computer program won hands down in making the measurement. A study dealing with measurement of ultrasound speckles is not one that most physicians and scientists are longing to read. However, measurement of speckles allows ultrasound to have much greater clinical relevance, from detecting cardiac stunning to evaluation of fistulas during angioplasty procedures.4,5 One limitation of speckle measurements in the past is that detecting speckles has required special ultrasound instruments which record the high frequency ultrasound signal directly into a file. The programs which the authors of this paper have developed and tested can be applied to ultrasound images which are recorded by standard ultrasound machines and kept in the DICOM files of every medical center. For patients undergoing vein mapping in preparation for fistulas, the radiologist or interventionalist will soon be able to apply the program to automatically determine the distensibility of the veins and arteries. Distensibility of veins and arteries will be a helpful feature in trying to determine which veins and arteries will mature if a fistula is created. However, compliance of the vessel will be a better indicator. To obtain the actual compliance of the vessel, it will be necessary to know the pressure change within the vessel with each heartbeat. The pulse pressure from a blood pressure cuff measurement will work fairly well for calculating compliance (elastic modulus) of an arm artery. Knowing the exact pressure change in a vein during cardiac systole will be a bit more challenging. However, there are probably ways to do this that we have not heard of yet.