It was often hard to imagine the far-fetched applications of basic physics when topics as humble as Acoustics, Optics and Magnetism were introduced in our high school physics textbooks. And it seems enthralling now to fathom how some of these basic disciplines have been applied for the development of some of the most sophisticated medical technologies of today’s world. Out of this fascination we decided to have a look at some of them briefly –
Optical Coherence Tomography
Optical coherence tomography (OCT) is an emerging technology for performing high-resolution cross-sectional imaging. OCT is analogous to ultrasound imaging, except that it uses light instead of sound. OCT can provide cross-sectional images of tissue structure on the micron scale in situ and in real time. OCT can function as a type of optical biopsy and is a powerful imaging technology for medical diagnostics because unlike conventional histopathology which requires removal of a tissue specimen and processing for microscopic examination, OCT can provide images of tissue in situ and in real time. By using the time-delay information contained in the light waves which have been reflected from different depths inside a sample, an OCT system reconstructs a depth-profile of the sample structure. Three-dimensional images can then be created by scanning the light beam laterally across the sample surface. Lateral resolution is determined by the spot size of the light beam whereas the depth (or axial) resolution depends primarily on the optical bandwidth of the light source. For this reason, OCT systems may combine high axial resolutions with large depths of field, so their primary applications include in-vivo imaging through thick sections of biological systems, particularly in the human body. The figure below shows a comparison of OCT resolution and imaging depths to those of alternative techniques; the “pendulum” length represents imaging depth, and the “sphere” size represents resolution (image source – UWA).
Elastography is based on the principle of physical elasticity which consists of applying a pressure on the examined medium and estimating the induced strain distribution by tracking the tissue motion. It uses the visualization of the propagation of mechanical waves through the tissue to derive either a shear wave velocity or a Young’s modulus as a measure of tissues stiffness. In practical terms, RF ultrasonic data before and after the applied compression are acquired and speckle tracking techniques, e.g., cross correlation methods, are employed in order to calculate the resulting strain. The resulting strain image is called an elastogram. The primary goal of elastography was the identification and characterization of breast lesions. To acquire an elastography image, the ultrasound technician takes a regular ultrasound image and then pushes on the tissue with the ultrasound transducer to take a compression image. Normal tissue and benign tumors are typically elastic or soft and compress easily whereas malignant tumors do not depress at all. The image below shows a traditional ultrasound image and a corresponding real-time elastogram of an ablated lesion in an ex vivo liver. In the elastogram, blue corresponds to hard tissue and red corresponds to soft tissue. The lesion is not clearly visible in the traditional ultrasound image because the ablation process does not change the echogenicity of the tissue significantly. However, the lesion is clearly visible in the elastogram (dark blue area) because the ablation process hardens the tissue significantly. Image Source-TAMUS.
Magnetoencephalography (MEG) is a non-invasive technique used to measure magnetic fields generated by small intracellular electrical currents in neurons of the brain. It allows the measurement of ongoing brain activity on a millisecond-by-millisecond basis, and it shows where in the brain activity is produced. MEG measurements are conducted externally, using an extremely sensitive device called a superconducting quantum interference device (SQUID). The SQUID is a very low noise detector of magnetic fields, which converts the magnetic flux threading using a pickup coil into voltage allowing detection of weak neuromagnetic signals. Since the SQUID relies on physical phenomena found in superconductors it requires cryogenic temperatures for operation. Due to low impedance at this temperature, the SQUID device can detect and amplify magnetic fields generated by neurons a few centimeters away from the sensors. A magnetically shielded room houses the equipment, and mitigates interference. Applications of MEG include localizing regions affected by pathology before surgical removal, determining the function of various parts of the brain, and neurofeedback.
Watch out this space as we deep dive into some of these technologies in greater detail and explore the rapidly evolving medical technology landscape!