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Angle dependency is one of the most often cited, yet less explored limitations of ultrasound in the measurement of TSL. If the standard pulse-echo technique is used, then an echo will only be detected from a surface or boundary when its path is similar to that of the original beam. In other words, a significant deviation from 90º incidence will lead to the echo being re-directed and not detected. Precisely how rapidly the echo strength decays as the angle changes depends on the topography and roughness of the surface and hence the relative amount of scattering involved (Wichard et al., 1996). The non-planar nature of human teeth limits the manoeuvrability of the ultrasound beam and consequently, measuring the thickness of enamel becomes more difficult. This has been mentioned in several research papers (see section 1.3) and is often described as an inherent limitation dictated by the shape of human teeth.

Incisors are obviously more planar than the remainder of the teeth in the arch and therefore might be considered as attractive targets for ultrasound investigation. They are prone to acid attack, from acidic fizzy drinks for example (unless a straw was used, passing the anterior teeth) and therefore can act as a reference for monitoring the acidic intake for patients. Furthermore, incisors are most vulnerable in bulimics and GORD patients, especially on the palatal surface, but access in vivo might be problematic. It might be predicted that other teeth (premolars and molars) may have

a different angle dependency to incisors, however this does not seem to have been the focus of much research.

It is not clear whether a reliable measurement of enamel thickness requires the acquisition of an image. Some studies have used B-mode imaging (Culjat et al., 2003; Harput et al., 2011) to measure enamel thickness, but this seems to have been limited to in vitro work. One of the earliest efforts to produce a B-mode image of a tooth (to measure bone levels around the tooth) was performed by Fukukita et al. (1985). The B-mode images, however, revealed the crown, gingiva and alveolar bone but not enamel thickness. This work was replicated by Berson et al. (1999), where they imaged a tooth and its periodontium, highlighting the potential of ultrasound in diagnosing periodontal disease by measuring alveolar bone height.

In an attempt to develop an ultrasound image of a tooth to quantify enamel layer thickness, Hua et al. (2009) used an ultrasound (13 MHz) medical scanner to image a molar tooth. The raw B-mode image required image processing to enhance it. Both images in Figure 1.11 below are not of adequate resolution to obtain enamel thickness measurements.

Figure 1.11. B-mode image of a molar before image processing, ‘A’ and after image processing, ‘B’ (Hua et al., 2009, p.441).

Another study examined the ultrasound (35 MHz) resonance method to obtain a 3D image of the enamel layer (Hughes et al., 2009). The study reported good results

with a 3D image of a tooth, however the study used a sectioned tooth specimen that was conditioned by polishing. This method is relatively ‘destructive’ and cannot be translated to the clinical level.

Whether or not a B-mode image is beneficial when making an ultrasonic scan of enamel thickness is an intriguing question. B-mode images can be saved on a computer and compared with future B-mode images to assess the level of erosive TSL that occurred. However, as enamel has a high SOS (see section 1.2, page 3), several echoes are generated within the tooth that obscure ‘real’ enamel layer echoes, which renders B-mode imaging more challenging. Nevertheless, using high frequency ultrasound improves axial resolution and therefore enhances B-mode images (discussed in Chapter 4).

Quantifying the thickness of the enamel layer would certainly help in monitoring erosive TSL. Enamel thickness measurements have been made by several researchers with A-mode ultrasound (see section 1.3.2). The reproducibility and accuracy of the measurements were often the two main outcomes reported. However, most of the reported results were from in vitro studies, mainly on extracted human teeth.

An in vitro A-mode study carried out in the Netherlands and Sweden (Louwerse et

al., 2004) tested the reproducibility of a 15 MHz ultrasonic device in measuring

enamel thickness on 12 anterior teeth across four observers. They concluded that there was high inter-observer variability in measurements (between baseline and repeat measurements) and that a thickness of less than 0.33 mm could not be reliably detected. The authors attributed this to poor reproducibility and probe positioning. However, they did not attribute the variation to the two inexperienced observers who only had two-hours of training in ultrasound measurements and waveform

recognition beforehand. In the same study, a reproducibility test was undertaken for the placement of the delay line type ultrasonic transducer. This was achieved by taking digital photographs of an ink-marked area (one-third of the tooth length from the gingival margin and one-half of the tooth width) of the tip of the ultrasound probe. This experiment was repeated after one week to test probe positioning reproducibility between two observers, and how that might impact enamel thickness measurements, but the variation in probe positioning between the two observers was negligible.

A similar in vitro study (Huysmans and Thijssen, 2000) to measure enamel thickness on extracted human incisors was successful but with limitations. Of note was the difficulty in obtaining a recognisable ultrasonic waveform from cervical areas on both buccal and palatal surfaces due to the very thin enamel layer (< 0.5 mm) and the non-planar surface at these sites (Huysmans and Thijssen, 2000). The problems caused by the curvature of the teeth in these areas were compounded by holding the tooth in one hand and the ultrasonic probe in the other. The authors pointed out that if these measurements were to be made in vivo, probe alignment and measurements would be easier. Nevertheless, the reproducibility (intra- and inter-examiner agreement) was good with intra-examiner limits of agreement for the first examiner (n = 20) at -0.064 to 0.061 mm and -0.084 to 0.061 mm for the second examiner. The inter-examiner limits of agreement (n = 42) were -0.09 and 0.09 mm. The majority of the studies investigating enamel thickness assumed a constant SOS in enamel (see section 1.3.2). This assumption produces uncertainties of the enamel thickness results obtained using a constant SOS, because SOS varies within the same tooth and across teeth (Slak et al., 2011). To assess the accuracy of the ultrasonic system, the SOS for

each tooth and each section of the tooth must be obtained first, and then the thickness of each section is derived and compared with histology (discussed in Chapter 5).

Bozkurt et al. (2005) investigated the accuracy of an ultrasonic system in detecting progressive occlusal wear on 20 human premolars in vitro. Thickness measurements were obtained using an ultrasonic system with a frequency of 11 MHz. The authors selected occlusal areas with some wear to guarantee a planar surface, which was scanned with a delay-line ultrasonic probe, with a tip diameter of 1.5 mm. The study reported a good agreement between ultrasound thickness measurements and histological sections of the teeth examined under a stereo microscope, as well as good inter-examiner reproducibility.

Because TSL is a progressive phenomenon (Lee et al., 2012), locating planar areas on occlusal surfaces to measure enamel thickness in vivo might be challenging, as fixed reference points are lost in erosive TSL (Mitchell et al., 2003). Also, the same planar reference that served as a baseline would not be present in successive scans. The value of an in vivo investigation of A-mode ultrasound reproducibility on teeth with naturally planar surfaces becomes very important, as these surfaces can serve as baselines for monitoring the condition. This would help determine how beneficial ultrasound can be in monitoring erosive TSL.

One of the main problems of applying diagnostic ultrasound in dentistry is coupling, which is required to transfer the ultrasound energy into the tooth and back to the transducer (see section 1.2). The outer surface of a tooth (enamel) is full of porosities (Crabb, 1976) which contain air. Air is acoustically ‘unfriendly’ and does not transmit ultrasound waves; ultrasound does not pass through air because of the impedance mismatch between the highly dense enamel and the less dense air (Harput

et al., 2011). On the microscopic scale, these porosities decrease the amount of

contact (surface area) between the transducer and the enamel surface (Lempriere, 2002). Therefore, a couplant such as water could be used to inhibit this effect.

Several research groups investigated the use of couplants, such as glycerine, castor oil, aluminium and mercury (mercury would not be used in an in vivo situation because of its toxicity). The aim of these investigations was to find the best coupling agent that would transmit most of the ultrasound wave with minimal loss of signal. This is done by selecting a coupling material with an impedance value that lies between the impedance values of the ultrasound wave source (transducer) and the tooth (enamel). Ideally it should be a solid, but that is clinically impractical.

Aluminium has an impedance value that is similar to enamel (Lees and Barber, 1968) and the piezoelectric transducer; theoretically, it is an ideal couplant. However, the use of aluminium is not practical in a clinical environment because it requires a perfectly planar surface to achieve good contact and thus coupling with enamel. Therefore, it is useful to know what materials are suitable for coupling transducers to teeth. This will be investigated in more detail in Chapter 2.

It would be sensible, therefore, to establish clinical reproducibility in an in vivo situation, which would pinpoint any limitations that might arise in a clinical situation and inform future research in this field (discussed in Chapter 6). Initial work to look at the possibility of identifying interfaces would also be valuable as enamel thickness is of central importance in the monitoring of TSL.

The aim of this thesis is to assess the feasibility and optimisation of ultrasound as a potential clinical dental tool aiding in quantifying the enamel layer to assess the possibility of using this tool in monitoring erosive TSL in vitro and in vivo.