Basal cell carcinoma (BCC) is the most common form of cancer in white populations across the world, with over 1 million new occurrences in the USA each year alone.  Typically, to study the extent of superficial BCC, a biopsy is taken of tumor and histology is performed.  Accurate determination of tumor boundaries is vital prior to removing the infected area. 

There are several techniques to image BCC.  High frequency ultrasound is capable of visualization tumor dimensions with 80 μm and 200 μm axial and lateral resolution, respectively.  However, it does not have chemical specificity and thus cannot differentiate between benign and malignant skin lesions.  Magnetic resonance imaging (MRI) can be used as well but the high cost, system complexity, acquisition times, and full body machine hinder its usability.  Near infrared (NIR) imaging techniques such as optical coherence tomography (OCT) and confocal microscopy can also be used however their poor penetration due to scattering of light means there is a loss of signal after a few hundred microns. 

Terahertz pulse imaging (TPI) is a non-invasive, coherent, optical imaging modality that works with wavelengths in the range of 3 mm to 30 μm.  Woodward et al. suggests a system with lateral and axial resolutions of 350 μm and 40 μm, respectively.  A schematic of the TPI system is as follows:

AO is the acousto-optic modulator, OAP is the off-axis parabolic mirrors, WP is the Wollaston polarization quarter waveplate, and ZnTe is zinc telluride. 

Using the above system, TPI was used to acquire images of excised regions of BCC.  The results are as follows:

The left column contains three different samples of both diseased tissue (solid boundary) and normal tissue (dashed boundary).  The rightmost column shows a color mapping of the terahertz pulse images.  Regions of interest d1, d2, n1, and n2 denote areas extracted for statistical analysis.  The mean time post pulse values of these regions are graphed in the center column. 

The change in absorption between diseased and healthy tissue provide a basis to use terahertz imaging. The results shown clearly demonstrate the ability to differentiate between healthy and diseased tissue with high specificity using the terahertz pulse imaging system.  This technique needs further investigation for the exact mechanisms of carcinogenic matter scattering properties but can and will ultimately be used as an in vivo technique for monitoring skin cancer.