The following article appeared in Med-Tech Innovation, May/June 2014
Advances in Ultrasound Imaging: GUIDED SURGERY
Paul Galluzzo of TTP charts the developments in miniature on-board ultrasound and highlights future applications of the technology and how the technical challenges are being overcome.
Ultrasound imaging has traditionally been associated with use in pregnancy, reporting the development of the foetus throughout the gestational period. But today the technology is used by many more medical teams; in many cases it replaces optical approaches such as cameras, which have limited use inside small intricate organs and tissues.
By retrofitting a miniature ultrasound probe to existing surgical tools, clinicians can transmit acoustic pulses into the tissue or organ and use the back-scattered acoustic waves to form the ultrasound image. This vision greatly improves surgical accuracy, resulting in a reduced chance of needing to repeat the surgery later and increasing patient safety because the risks are observed and managed throughout the procedure.
In theatre, miniature on-board ultrasound technology gives the surgeon a clearer and more detailed image of the organ or tissue that they are operating on. This is the case in the treatment of atrial fibrillation, an irregular heartbeat that is often treated by radiofrequency (RF) ablation of unwanted electrical conduction pathways in the heart wall.
The integration of ultrasound imaging and RF ablation into a single device means surgeons can detect the amount of RF ablation that is needed while the patient is in theatre. This reduces re-intervention rates and the number of deaths associated with excessive ablation.
Onboard ultrasound is also being successfully used in the investigation of tissue abnormalities. A 2-mm diameter ultrasound device is incorporated into a catheter alongside debridement or ablation technology, without compromising the performance of the latter. With a 2-mm acoustic aperture, operating at 15 MHz, the ultrasound probe provides an image of internal features located up to 1 0-mm below the surface of the tissue.
This enables different types of tissue to be identified by geometric patterns of varying echogenicity associated with anatomical features. For example, with knowledge of the local anatomy a surgeon can identify tumours in an ultrasound image, or landmarks that help the surgeon navigate through the vasculature to a required location. In other cases the surgeon may look for more subtle features in the image, for example necrotic ablated tissue can sometimes be identified due to being more hypoechoic than surrounding healthy tissue.
In the case of oncology, catheter-mounted imaging technology may even provide enough information for the clinician to identify tumours hidden behind acoustically opaque zones, without the need to take a biopsy. With knowledge of the local anatomy, the diagnosis may be possible based on a series of ultrasound images alone.
In the future these devices can and should be used for so much more. For example, they will be used in the diagnosis and treatment of peripheral vascular disease, through integration with atherectomy catheters.
Atherectomy is an exciting and growing method of treating peripheral vascular disease, whereby plaque is physically removed from a blood vessel, rather than pushed sideways using balloon angioplasty or covered or bypassed using a graft or stent. This removal technique is generally less traumatic and leaves no foreign object behind. The integration of on-board ultrasound imaging with atherectomy in a single catheter works for two reasons. First, the clinician can navigate to blood vessel blockages without having to swap between multiple catheters; and second, it enables the clinician to monitor the progress of the atherectomy procedure in real time and ensure adequate but not excessive tissue removal.
The main technical challenge presented by peripheral intravascular applications is the tiny diameter of catheter required for access. The key to tackling this challenge is to simplify the mechanical and electrical configurations as much as possible while still tailoring the imaging capabilities around the application. For example:
• A single element ultrasound probe is sufficient to produce a high resolution "A scan" of surrounding tissue, but avoids multiple electrical connections or on-board multiplexing electronics associated with phased array imaging.
• Acoustic matching layers improve coupling from ultrasound probes into the tissue, resulting in a level of sensitivity that is sustained across a broader range of frequencies. By committing to a single operating frequency such as 20 MHz, instead of seeking a broad bandwidth, it is possible to operate with a reduced thickness-matching layer to save space and still achieve sensitivity comparable with thicker matching layer probes.
• Many standard image processing algorithms used in ultrasound imaging have been optimised for use on "B-scan" data, whereas fewer elements operating at a higher frame rate presents an opportunity to tailor the approach for computational efficiency and performance.
Detection of "vulnerable plaque"
Another frontier in the management of heart disease is in the detection of "vulnerable plaque," a frequent precursor to heart attacks; miniature on-board ultrasound will play a key role in this. Vulnerable plaque refers to cases where fatty deposits in arteries are held in place only by a fibrous cap; under physical stress, the cap ruptures and spills the contents into the bloodstream.
Detection of the telltale fibrous cap is important to the patient's prognosis, but is hard to see with ultrasound alone. However, the fibrous cap's absorption of infrared light significantly contrasts the surrounding material, and this can be exploited by the use of photoacoustic imaging, that is, detection of the acoustic waves caused by thermal expansion following optical absorption in the first couple millimeters of tissue. In the future, there will be photoacoustic-imaging catheters that will detect fibrous caps and then evaluate the risk of plaque rapture.
Barriers to adoption
Until now advances in ultrasound imaging technology, from obstetrics onwards, have been held back because commercially viable innovation requires a combination of technological expertise, user centred design and business enterprise.
Centres of excellence for medical imaging can lack the agility to address new markets with new solutions, often viewing these applications as too niche. And conversely, the major surgical tool manufacturers often lack a core competency in technologies such as ultrasound imaging to realise what is possible. It is this "gap" that has enabled TTP to develop sophisticated surgical guidance technology, which addresses real clinical needs.
Mass-market ultrasound imaging is getting more mature and is now at the point where manufacturers compete on cost and application-specific usability rather than new technology. However, market adoption of any new miniature on-board imaging devices has to be costeffective. With good design, for example, placing the emphasis on simple manufacturable assemblies, the cost of the disposable surgical tool/imaging catheter can be close to the cost of the original non-imaging surgical tool. The rise in cost of the disposable is thus minimised and far outweighed by the value of the improved clinical outcome.
Furthermore, not only can the cost of the disposable be kept low, the supporting equipment can be produced at low cost as well: with increasing commoditisation
of the instrumentation that provides control and signal processing, the console itself can be produced for less than $1000 for an ultrasound imaging system, which means that the customer does not need to bear a large capital expenditure.
When surgeons are operating on patients it is crucial for them to be able to see what they are doing more clearly. In theatre, miniature on-board ultrasound technology gives medical teams a more detailed image of the organ or tissue that they are operating on. This makes diagnosis and repair easier, more thorough and reduces the chance of repeat operations.
With the critical technology at tipping point, TTP is excited to play a role in the democratisation of medical imaging that will improve healthcare across the world.