A Comparison of Terahertz Pulsed Imaging with Transmission Microradiography for Depth Measurement of Enamel Demineralisation in vitro

Abstract

Terahertz pulsed imaging (TPI) is a relatively new, non-ionising and non-destructive imaging technique for studying hard tissues which does not require tooth section preparation, unlike transmission microradiography (TMR). If TPI can measure the depths of caries/demineralisation lesions accurately the same tooth samples could be reused and remeasured during in vitro and in situ studies on de- and/or remineralisation. The aim of this study was to compare TPI and TMR for measuring the depths of a range of artificially induced bovine enamel demineralised lesions in vitro. Bovine slabs with artificial caries, induced to different levels of demineralisation by two different but standard demineralisation techniques (‘acid gel’ and ‘carbopol’) were measured by TPI and TMR and the readings compared. The set of TPI/TMR measurements obtained on the gel-demineralised slabs showed an extremely high coefficient of determination (r² = 0.995). Detailed analysis of the results and theoretical considerations (involving the relationship between refractive index profiling and mineral loss profile) are used to explain the findings and show that for acid gel lesions TPI is measuring demineralisation in the range of 47% of that of TMR depth plus an intercept of 16 µm, with further calculations allowing the TMR depths to be determined to within 5% using TPI.

Spectroscopic Study of Human Teeth and Blood from Visible to Terahertz Frequencies for Clinical Diagnosis of Dental Pulp Vitality

“…Optical methods appear to offer a reliable assessment of pulp vitality. Much work has already been carried out in the field of imaging and caries localization by throughput of light with an emphasis on the determination of suitable transmission windows in the IR and THz spectral ranges [7][8][9][10][11][12].…”

Diagnostic Imaging and Spectroscopy

“…Crawley et al [165] showed that areas of demineralized and hypomineralized enamel can be discriminated from sound tissue in TPI images. The depth of artificial surfacesoftened lesions created in enamel using an acid gel measured by TPI compared well with measurements [167]. These methods appear best suited for measurement of the lesion depth and the remaining enamel thickness for erosion measurements.…”

Freeze-thaw hysteresis effects in terahertz imaging of biomedical tissues

“…Thus, the slow-freeze, slow-thaw cycle will reduce the THz contrast in cases such as muscle-fat comparisons as the refractive index of the muscle will become lower and therefore closer to that of the fat. However the contrast will be improved in cases where the biomedical material of low water content has a higher refractive index such as enamel (n = 3 at 1 THz [8]): in such cases, after the freeze-thaw process, the muscle will have a lower refractive index and so the difference between the muscle and enamel will be greater. Thus, for inhomogeneous samples, it will depend on the composition of the inhomogeneities as to whether the contrast will be reduced or enhanced.…”

“…Several studies have proposed interesting uses of THz waves such as to delineate cancer margins [2][3][4], monitor the water content in living plants [5,6], and detect dental defects [7,8].…”