Measurement of resolution in intravascular ultrasound images

Detailed images of arteries can be formed by inserting intravascular ultrasound (IVUS) catheters within them. These catheters contain rotating ultrasound transducers and are capable of displaying disease and features such as the layers of artery walls. There is, however, relatively little information available on IVUS resolution even though this is necessary to interpret the images. Techniques for assessing IVUS resolution in three dimensions are described for a 20 MHz Boston Scientific catheter and a Diasonics IVUS scanner as well as the results obtained. Out-of-plane resolution was measured by finding the ultrasound beam thickness. Axial and lateral resolution were assessed by the minimum separation of two point targets when their images could just be distinguished. The out-of-plane resolution was found to improve slightly with distance from the catheter, varying from 2.0-1.5 mm. Lateral resolution worsened in a linear manner with distance. At a distance of 5 mm from the catheter, lateral resolution was 0.83 mm. The axial resolution was the best of the three resolutions and was actually limited by the screen display scale. With the highest magnification the axial resolution was approximately 0.19 mm and was constant with distance.