| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ||||||||||||||||||||||||||||||||||||||||||||||||||
Commentary |
Department of Radiology, Freeman Hospital, High Heaton, Newcastle upon Tyne NE7 7DN, UK
Correspondence: S T Elliott, Department of Radiology, Freeman Hospital, High Heaton, Newcastle upon Tyne NE7 7DN, UK. E-mail: simon.elliott{at}nuth.nhs.uk
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 non-diagnostic 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 on-line 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:
The same post-processing software can be applied to both types of acquired volume datasets and, therefore, the ultrasound practitioner must think of manipulating ultrasound images in new and different ways. For example, an acquired volume might include one whole kidney; measuring the renal length accurately is simply a matter of rotating one of the MPR planes until the maximum diameter is displayed, and then performing calliper placement in the usual way. This technique has been shown to be as accurate as conventional single plane measurement in the abdomen [4]. If the volume is stored, such measurements can of course be reviewed or repeated at any time, and by anyone. As a technique recognized as having negligible physical risk, ultrasound has a long-established role in the surveillance of many conditions, e.g. abdominal aortic aneurysm, chronic renal disease and the follow-up of known focal mass lesions. These examinations may involve simple measurements or a more complex, and possibly subjective, assessment of changes in tissue character. The availability of a volume dataset for the target abnormality, in perpetuity, will bring the practice of such ultrasound examinations into line with CT and MRI.
The speed of volume acquisition is a significant advantage, notably in paediatric practice. The neonatal brain can be imaged rapidly by trans-fontanelle volume scans, and early reports of the potential of this technique have been confirmed by Fritz et al [5]. Imaging time at the bedside on a neonatal intensive care unit was reduced from a mean of 9.1 min (two-dimensional (2D)) to 4.8 min (3D); they also found that standardization, documentation and follow-up were also improved by the availability of volume data. Efficiency gains are already being shown by the increasing use of volume ultrasound in obstetric practice. Benacerraf et al [6] report a remarkable reduction in examination time from a mean of 19.6 min (2D) to 1.8 min (3D) for a fetal anatomical survey at 17–21 weeks' gestation. Interpretation of the 3D images took approximately 7 min; any differences in identifying anatomical landmarks and in measurements between the two techniques were found not to be clinically significant.
The use of volume data for measurement (volumetric ultrasound) is likely to speed and improve our assessment of complex anatomical and pathological structures. The most widely used volume measurement currently is the "three diameter ellipsoid" method, e.g. in estimating bladder and gallbladder volumes. This is a time-consuming procedure, usually performed while the patient is on the couch, and it carries an inherent error that is usually quoted at around 25%. By using volume ultrasound and stacked contour methods, some of which are now semi-automated, more accurate measurements can be obtained, particularly of complex non-ellipsoid structures. Preliminary in vitro investigation in our own department showed that this method not only reduced volume errors from around 23% to 11%, but also reduced interobserver variations, an important factor in sequential examinations.
The working practice in many UK centres and in most of the United States involves radiologists "secondary" reporting from still images of single-scan planes acquired by a sonographer. One major criticism of conventional ultrasound, from referring clinicians and from those involved in training and supervision, is the uncertainty that the operator might have missed pathology or performed an incorrect measurement, because no image was acquired in the appropriate scan plane. Although standard protocols exist for scan plane selection, there is always the risk, and a nagging worry, that pathology lies "just next door" and was missed during the examination. Volume ultrasound is likely to remove this uncertainty, especially from the minds of clinicians; "this was missed by ultrasound" is a phrase commonly heard in multidisciplinary meetings, but one which is rarely applied to other cross-sectional modalities. The training of ultrasound practitioners is affected by the real-time nature of conventional imaging. Trainers must be present during the entire examination, repeat the examination themselves, or perhaps review the entire procedure from a recorded video, all of which have major implications for the staffing of a teaching department. Volume ultrasound allows the trainer to review, analyse and discuss the findings at leisure, potentially improving efficiency.
Yet despite the probable advantages of volume ultrasound, not everyone is excited by its arrival. Ultrasonography is, and probably always will be, a skilled profession, with that skill applied almost entirely during the stage of image acquisition, i.e. the examination. A good ultrasound practitioner will operate a complex scanning system as if playing a musical instrument, while at the same time making diagnostic decisions based upon interpretation of the displayed dynamic images. However, a trend among all manufacturers is to make ultrasound systems increasingly "intelligent": image optimization can be achieved instantaneously by a one-touch button, and by real-time adaptive image processing working constantly in the background. As such, some of the control skills are already being progressively taken away from the operator and, not surprisingly, some sonographers express concern that their skills could be further eroded by volume ultrasound, reducing the examination to a series of quick, automatically optimized volume acquisitions.
All imaging modalities progress by a steady evolution, interspersed with discrete technological step-wise changes. For ultrasound, these steps would include grey-scale, real-time, colour Doppler and microbubble contrast imaging. If embraced by the major manufacturers, and provided that the necessary equipment migrates into non-obstetric imaging departments, volume ultrasound has the potential to be the next significant step. Further evaluation of this promising technique is required, but already there are proven benefits for diagnostic confidence, accuracy and changes in clinical practice.
The author has received financial support for conference travel from Philips Medical Systems. No other financial or contractual interest.
Received for publication January 15, 2007. Revision received January 16, 2007. Accepted for publication January 22, 2007.
 |
References |
|---|
 Top References |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| BJR | DMFR | IMAGING | ALL BIR JOURNALS |
