A pulsed terahertz imaging system has been developed for potential use in vivo. Few data are available regarding the optical properties of human tissue at terahertz frequencies. This work demonstrates
transmission measurements through human ex vivo tissue sections, and determines broadband refractive indices, and broadband and frequency dependent absorption coefficients. The data presented here are the first systematic measurements of this type. Significant differences were found between a numbers of human tissue types.
Terahertz frequency spectroscopic imaging studies of teeth are reported. The aim is to establish the characteristic properties of enamel and dentine at these high frequencies. Changes to the THz characteristics as a result of various types of tooth abnormalities are reported showing the potential of this technique for dental diagnosis
The first demonstrations of terahertz imaging in biomedicine were made several years ago, but few data are available on the optical properties of human tissue at terahertz frequencies. A catalogue of these properties has been established to estimate variability and determine the practicality of proposed medical applications in terms of penetration depth, image contrast and reflection at boundaries. A pulsed terahertz imaging system with a useful bandwidth 0.5-2.5 THz was used. Local ethical committee approval was obtained. Transmission measurements were made through tissue slices of thickness 0.08 to 1 mm, including tooth enamel and dentine, cortical bone, skin, adipose tissue and striated muscle. The mean and standard deviation for refractive index and linear attenuation coefficient, both broadband and as a function of frequency, were calculated. The measurements were used in simple models of the transmission, reflection and propagation of terahertz radiation in potential medical applications. Refractive indices ranged from 1.5 ± 0.5 for adipose tissue to 3.06 ± 0.09 for tooth enamel. Significant differences (P < 0.05) were found between the broadband refractive indices of a number of tissues. Terahertz radiation is strongly absorbed in tissue so reflection imaging, which has lower penetration requirements than transmission, shows promise for dental or dermatological applications.
Terahertz (THz) radiation is being studied as an investigative tool for skin conditions. Two approaches for describing the propagation of THz radiation through skin are presented and verified using a layered water-based phantom. The skin was assumed to comprise a series of layers of tissue with differing, frequency dependent, properties; the major interaction was assumed to be between THz radiation and water. Based on these assumptions a thin film matrix model and a Monte Carlo model were developed to simulate this situation. In order to test these models, a simple three layer in-vitro phantom was used. This consisted of two 2 mm layers of TPX, encasing a 180 micrometer layer of a water/propanol-1 mixture. Spectroscopic measurements were made in a pulsed THz system for cells with thirteen different water/propanol-1 concentrations. Comparisons between the results from both models and experimental spectra show good correlation, in each case the model was able to simulate the overall trend of the spectra and more detailed features. This suggests that the models may be adapted to investigate THz irradiation of skin. Modeling modifications would include using layer dimensions that were comparable to the constituent layers of skin and using additional layers to describe the organ more thoroughly.
Terahertz (THz) radiation has a frequency of the order of 101212 Hz. This lies between the infrared and microwave regions of the electromagnetic spectrum; a section labeled the 'THz gap'. Infrared and microwave radiation is used in the medical field; research is underway for an application for THz radiation. At present no formal safety analysis of a THz pulsed imaging (TPI) system has been performed. This will be necessary for future in vivo studies. The radiation is delivered in a train of femtosecond pulses. International guidelines on exposure to non-ionizing radiation, and supporting literature, were reviewed to determine the Maximum Permissible Exposure (MPE) for radiation of this range of wavelengths, both for a single pulse and continuous wave exposure. Two methods of deriving the MPE were identified. Current guidelines for infrared and microwave regions of the electromagnetic spectrum incorporate the THz region. Using conservative parameter estimation an MPE per pulse, over the area of the beam, of 94 W was calculated. At present THz pulsed imaging systems produce pulses of power approximately 1 mW; this lies within the limit calculated using the published guidelines. There are, however, areas requiring further investigation before the technique becomes routine in clinical practice.
Terahertz imaging is an emerging modality, with potential for medical applications, using broadband sub-picosecond electromagnetic pulses in the range of frequencies between 100 GHz and 100 terahertz (THz). Images can be formed using parameters derived from the time domain, or at the range of frequencies in the Fourier domain. The choice of frequency at which to image may be an important factor for clinical applications. Image quality as a function of frequency was assessed for a terahertz pulsed imaging system by means of; (i) image noise measurements on a specially designed step wedge, and (ii) modulation transfer functions (MTF) derived from a range of spatial frequency square wave patterns. It was found that frequencies with larger signal magnitude gave lower image noise, measured using relative standard deviation (standard deviation divided by mean) for uniform regions of interest of the step wedge image. MTF results were as expected, with higher THz frequency signals demonstrating a consistently higher MTF and higher spatial frequency limiting resolution than the lower THz frequencies. There is a trade-off between image noise and spatial resolution with image frequency. Higher frequencies exhibit better spatial resolution than lower frequencies, however the decrease in signal power results in a degradation of the image.
KEYWORDS: 3D modeling, Image segmentation, Data modeling, Arteries, 3D image processing, Image processing, Distance measurement, Visual process modeling, Systems modeling, Shape memory alloys
A self-optimizing 3D deformable model has been developed which is able to segment branching anatomy. Its performance is compared with that of interactive segmentation in spiral CT of abdominal aortic aneurysms. SCT data from six individuals were selected retrospectively, representing a range of vascular geometry and tortuosity. As a reference, segmentation was performed twice by one observer using interactive 2D region growing. The self-optimizing 3D deformable model was applied twice, each with different initializations of the model in the aortic lumen. Dimensional and volume measurements were made, and boundary positions compared. The model was found to give qualitatively good representation, but was not able to follow vessels distal to the iliac bifurcation. The results agreed very well with the 2D interactive technique where structures ran orthogonal to the slice plane, with structures localized to 0.5 mm. The percentage difference in volume estimation between the model and the reference was 3% (the same as the agreement between the two reference segmentations). The mean closest distance between model and reference boundaries was 1.2 +/- 0.5 mm. Most discrepancies occurred at the bifurcations, and we conclude that the 3D deformable model requires further development for accurate representation of branching vascular structures in disease, but the accuracy of the model segmentation is sufficient for visualization or training.
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