Nickolay N. Zinov'ev

An introduction to medical imaging with coherent terahertz frequency radiation

Methods have recently been developed that make use of electromagnetic radiation at terahertz (THz) frequencies, the region of the spectrum between millimetre wavelengths and the infrared, for imaging purposes. Radiation at these wavelengths is non-ionizing and subject to far less Rayleigh scatter than visible or infrared wavelengths, making it suitable for medical applications. This paper introduces THz pulsed imaging and discusses its potential for in vivo medical applications in comparison with existing modalities.

Catalogue of human tissue optical properties at terahertz frequencies

Recently published studies suggest thatterahertz pulsed imaging will have applications inmedicine and biology, but there iscurrently very little information about the opticalproperties of human tissue at terahertzfrequencies. Such information would be useful forpredicting the feasibility of proposedapplications, optimising acquisition protocols,providing information about variability ofhealthy tissue and supplying data for studies of theinteraction mechanisms. Research ethicscommittee approval was obtained, andmeasurements made from samples of freshlyexcised human tissue, using a broadbandterahertz pulsed imaging system comprisingfrequencies approximately 0.5 to 2.5 THz.Refractive index and linear absorptioncoefficient were found. Reproducibility wasdetermined using blood from one volunteer,which was drawn and measured on consecutivedays. Skin, adipose tissue, striatedmuscle, vein and nerve were measured (to date, from oneindividual). Water had a higher refractiveindex (2.04 ± 0.07) than any tissue.The linear absorption coefficient was higher formuscle than adipose tissue, as expectedfrom the higher hydration of muscle. As these samples camefrom a single subject, there is currentlyinsufficient statistical power to draw firmconclusions, but results suggest that in vivo clinical imaging will be feasible in certainapplications.

Optical properties of tissue measured using terahertz pulsed imaging.

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.

Evaluation of image quality in terahertz pulsed imaging using test objects

As with other imaging modalities, the performance of terahertz (THz) imaging systems is limited by factors of spatial resolution, contrast and noise. The purpose of this paper is to introduce test objects and image analysis methods to evaluate and compare THz image quality in a quantitative and objective way, so that alternative terahertz imaging system configurations and acquisition techniques can be compared, and the range of image parameters can be assessed. Two test objects were designed and manufactured, one to determine the modulation transfer functions (MTF) and the other to derive image signal to noise ratio (SNR) at a range of contrasts. As expected the higher THz frequencies had larger MTFs, and better spatial resolution as determined by the spatial frequency at which the MTF dropped below the 20% threshold. Image SNR was compared for time domain and frequency domain image parameters and time delay based images consistently demonstrated higher SNR than intensity based parameters such as relative transmittance because the latter are more strongly affected by the sources of noise in the THz system such as laser fluctuations and detector shot noise.

Two methods for modelling the propagation of terahertz radiation in a layered structure.

Modelling the interaction of terahertz(THz) radiation with biological tissueposes many interesting problems. THzradiation is neither obviously described byan electric field distribution or anensemble of photons and biological tissueis an inhomogeneous medium with anelectronic permittivity that is bothspatially and frequency dependent making ita complex system to model.A three-layer system of parallel-sidedslabs has been used as the system throughwhich the passage of THz radiation has beensimulated. Two modelling approaches havebeen developed a thin film matrix model anda Monte Carlo model. The source data foreach of these methods, taken at the sametime as the data recorded to experimentallyverify them, was a THz spectrum that hadpassed though air only.Experimental verification of these twomodels was carried out using athree-layered in vitro phantom. Simulatedtransmission spectrum data was compared toexperimental transmission spectrum datafirst to determine and then to compare theaccuracy of the two methods. Goodagreement was found, with typical resultshaving a correlation coefficient of 0.90for the thin film matrix model and 0.78 forthe Monte Carlo model over the full THzspectrum. Further work is underway toimprove the models above 1 THz.

Optical properties of tissue at terahertz frequencies

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.

The optical properties of human tissue at terahertz frequencies

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.

Identification of tooth decay using terahertz imaging and spectroscopy

Terahertz (THz) frequency spectroscopic imaging studies of teeth are reported. The aim is to establish the characteristic properties of the enamel and dentine at these high frequencies. Changes to the THz characteristics as a result of various types of tooth decay are reported showing the potential of this technique for dental diagnosis.

Identification of tooth abnormalities using terahertz imaging and spectroscopy

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

What constitutes a useful and meaningful terahertz image

This contribution attempts to answer the questions of how THz images are best processed, analyzed, presented, classified and interpreted, especially within a clinical setting. The solutions to these problems are reviewed within the context of recent imaging work on tumours and dental tissue, largely conducted by the Teravision group. It is concluded that compound images, especially those involving pulse broadening as an image parameter, are highly promising routes to visualisation. The possibility of using computer vision and data compression techniques is also discussed.