Transmission Ultrasonography with old Cavitat®
TAU is well known in some technical fields such as the aircraft industry, e.g. for the examination of welding seams. But it is also used in the medical field for the determination of bone densities for the analysis of osteoporosis
(cf. C. M. Langton and C. F. Njeh. “The Measurement of Broadband Ultrasonic Attenuation in Cancellous Bone—A Review of the Science and Technology”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2008 (55) 7: 1546-1554.)
The old Cavitat® apparatus is believed to be the first disclosure of a TAU apparatus for detecting jawbone cavitations. Cavitat® was commercially available for a couple of years.
Jacques Imbeau describes in more detail the underlying principles and the work with this apparatus in the publication J. Imbeau, “Introduction to Through-Transmission Alveolar Ultrasonography (TAU) in Dental Medicine”, The Journal of Craniomandibular Practice, 2005 (23) 2: 100-112.
However, the Cavitat® shows considerable drawbacks which are explained in more depth by Imbeau, ibid. page 106/107. These drawbacks are:
- First, the correct and reproducible placement of the sensor and of the transducer – in respect to the anatomical structures involved – affords a considerable amount of skill and experience making the technique rather susceptible and difficult, so that i.a. Imbeau calls TAU a “user sensitive technique” (Imbeau, ibid. page 105). Correspondingly, the reproducibility of the measurements was rather low. Furthermore, the intricate problem of air bubbles in the ultrasonic gel used is already mentioned by Imbeau; air bubbles falsify in an unpredictable way the ultrasonic signals which pass through a medium, in this case through a jawbone.
- Second, the signals coming from the receiving elements of the Cavitat Ultrasonograph were very weak. The evaluation of these signals was therefore a very demanding task. Due to this the sensitivity of the TAU apparatus was not very great. Furthermore, the method of signal analysis is not disclosed in US 6,030,221A (cf. Imbeau, ibid. page 104).
- Third, the construction of the ultrasound receiving unit of the Cavitat Ultrasonograph was not sufficiently sturdy such that, during use, more of the individual measuring units of the receiving unit broke, whilst concomitantly spare parts were no longer available as the manufacturer, Cavitat Medical Technologies, had been out of business for some time. Hence, the technology became less accessible.
- Fourth, for the user there was no possibility to check the calibration of the TAU apparatus regularly over the course of time.
Characteristics of the new CaviTAU®
- First it is difficult to keep the ultrasonic gel used free from any kind of air bubbles, because of the high viscosity of the gel. Air bubbles inhibit reliable and repeatable measurements.
- Second the anatomical contour of the jawbones at the measuring site and the plane surface of the measuring unit (normally the receiver) do not fit at all. The distance between the surface of the measuring unit and the surface of the alveolar ridge varies widely, up to approximately 5 mm, so that large amounts of the coupling gel have to be applied to completely fill the gap and to get good acoustical conductivity in the entire measurement area, a fact which is expensive for the user and very unpleasant for the patient.
- Third, applying such large amounts of the gel again demands that the gel has to be kept completely free from air bubbles, a condition which is difficult to fulfill in reality.
- Fourth the gel has to stay in place but this happens normally only for a restricted period of time. Gradually the gel disappears somewhere inside of the mouth of the patient. Therefore the gel has to be replenished frequently; this creates even more the problem to keep the gel free from air bubbles because the visual inspection of the gel for the absence of air bubbles inside the mouth is very difficult.
- Fifth in case that accidentally the alveolar ridge is only partly covered by the gel and/or that accidentally air bubbles are entrapped within the gel, the old Cavitat® apparatus does not deliver any warning signs but continues to display ultrasonic images which however are pure artefacts without any pathophysiological meaning and of course being free from repeatability. The occurrence of artefacts and the absence of warning signs represent the most intricate problems of the TAU measurements up to now.