Volume ultrasound: the next big thing?

The term ‘‘3D ultrasound’’ is most likely to conjure up a mental picture of a baby’s face. Indeed, this probably represents the most public face of ultrasound in the early 21st century, with emotive images of the fetus even appearing on the front page of national newspapers in the UK. Such surface-rendered images are understandably popular with parents, and can provide a profitable ‘‘baby-bonding’’ business for private ultrasound services, but there is concern among both professional organizations and governing authorities about the use of three-dimensional (3D) ultrasound for purely nondiagnostic or ‘‘souvenir’’ purposes [1, 2]. The diagnostic value of surface-rendered 3D ultrasound in general non-obstetric imaging has been considered minimal, and few patients want a pretty picture of their gallstones or enlarged prostate to take away and show to relatives. However, the same technology that is capable of producing these 3D images also allows for volume acquisition, with subsequent online or off-line multiplanar reconstruction (MPR), multislice imaging and volumetric analysis. To the radiologist familiar with CT or MRI, where multislice imaging and MPR are the norm, these new and imaginative techniques could also offer considerable diagnostic potential for non-obstetric ultrasound. 3D ultrasound has been commercially available on premium scanning systems for several years by using a freehand sweep technique and subsequent image reconstruction. The success of this technique is limited by the operator skill needed to acquire the volume, by the time taken to complete the acquisition, and by the lack of volumetric accuracy. Dedicated volume acquisition systems now fall into two main categories, depending on the type of transducer employed: 1. Mechanical: a conventional multi-element transducer head, using a single scan plane, is mechanically swept from side-to-side over the region of interest. The acquired volume is displayed as a static image at the end of the sweep, or as a real-time 3D image (especially for surface rendering of the fetus). Acquisition time for a static image is approximately 3 s. At present, these transducers offer higher detail resolution, but this might only be a short-term advantage. 2. Matrix: complex new transducers featuring arrays of thousands of active elements. These offer near instantaneous acquisition of static volume datasets, thus reducing volumetric errors produced by respiratory and other motion artefacts. They reveal their evolution from cardiac applications by additionally providing live multiplanar ultrasound, real-time 3D ultrasound (i.e. four-dimensional (4D) ultrasound), and even 3D and 4D colour Doppler imaging. Also under development for matrix, and hence volumetric, imaging are capacitive micro-machined ultrasonic transducers (CMUTs). By combining existing silicon chip technology with revolutionary silicon membrane construction, CMUTs are extremely lightweight transducers with potentially hundreds of thousands of active elements [3].