Intravascular Ultrasound Imaging of Pulmonary Arteries: Limitations of Pulmonary IVUS
Despite the advantages of pulmonary IVUS imaging over other currently available techniques, there are limitations with this method that need to be overcome.
First, the proximal pulmonary arteries are large and often beyond the penetration of 12 to 30 MHz ultrasound frequencies: 5 to 20 mm depth penetration depending on the frequency used. Therefore, lower frequency catheters will be required to provide detailed evaluations of pulmonary vascular responses and wall abnormalities in these more proximal portions. Because these proximal pulmonary arteries are large, it is also difficult to maintain central catheter placement. Eccentric placement of the catheter may distort the measurements of vessel diameter, and methods to improve catheter stability in these larger vessels are needed. read
Second, although this technique is the only method of qualitatively determining the severity of vascular wall abnormalities, the ability to characterize the histologic components of the increased wall thickness has not been evaluated. Since the pulmonary arteries are elastic vessels, a three-ringed appearance is not usually seen. Thromboembolism has been described as “soft” echoes, but reflective differences between intimal hyperplasia and medial hypertrophy have not been described. Thus, it is too early to determine whether pulmonary IVUS can detect the components of pulmonary vessel wall disease.
A third limitation with pulmonary IVUS is that it is an invasive technique. It does not, however, require radiographic contrast, and the time period for the subjective evaluation of proximal and distal pulmonary artery structure and function has been shown to take only minutes to perform. Finally, the cost vs benefit ratio of performing pulmonary IVUS is unclear. Currently, it should still be regarded as an investigational tool. Larger studies assessing the additional clinical value obtained from pulmonary IVUS compared with conventional imaging techniques in pulmonary vascular diseases are necessary to determine its cost effectiveness.