Terahertz imaging detects cancerous tissue

Abstract

The terahertz (THz) frequency range lies between the infrared and microwave regions of the spectrum and corresponds to wavelengths from millimeters to microns. The associated low energies interact with low-frequency motions in molecular systems such as the flexing of individual molecules or intermolecular interactions through either weak van der Waals forces or the stronger hydrogen bonds. This interactivity makes THzfrequency radiation highly sensitive to water concentration as evident in the properties of soft tissues.1 THz radiation is nonionizing and not highly scattered in tissues (unlike optical emission), thus making it attractive for use in biomedical applications.2 The development in the last few years of practical methods to generate and detect broadband pulses of THz radiation that can be contained within portable systems suitable for clinical studies has led to its use in the detection of skin and breast cancers.3, 4 THz radiation is generated using a photoconductive emitter consisting of a small piece of semiconductor crystal (commonly gallium arsenide) on which two planar metal electrodes form an antenna supporting a large electric field across its surface (see Figure 1). Ultrafast (approximately 100fs) pulses of light (commonly from a titanium/sapphire laser at a wavelength of 800nm) are then focused onto the gap between the electrodes. This generates charge carriers in the form of electron–hole pairs. The application of a bias voltage accelerates these charge carriers, producing a THz pulse that is radiated into free space.5 A number of detection techniques can be used, including bolometric measurement, electrooptic sampling, and photoconductive receivers (with the latter essentially operating in reverse to photoconductive emitters). Cancer of the colon, which is part of the gastrointestinal (GI) tract, is the third most common cancer in both men and women and the third leading cause of cancer-related mortality in the USA. The number of newly diagnosed cases of GI canFigure 1. Schematic diagram showing the process of terahertz (THz)radiation generation. Vbias: Bias voltage applied to the galliumarsenide (GaAs) substrate.