Like so much of medicine, the world of medical imagining is full of arcane acronyms, big words, and very smart people tossing around phrases like ‘collinear equilibrium magnetization’ like they are ordering lunch. But despite its magnificent complexity, medical imaging is a part of almost every person’s life these days- whether it be for a prenatal ultrasound, a dental x-ray, or a brain MRI; and understanding the difference between the several modalities to find out how does medical imaging work and how each works can prove a powerful tool during your next hospital visit.
Let us begin with the impetus behind medical imaging: the ability to identify and diagnose medical problems non-invasively by looking through skin, subcutaneous fat, and muscle. Doctors can identify tumors, locate fractures, visualize organs and blood vessels, target blood clots, pinpoint implant locations and much, much more.
All imaging modalities are based on the analysis of a signal which is produced by a machine. The signal interacts with the body in a particular way and is then received and analyzed by the machine to generate meaningful information. Lets begin with perhaps the most basic modality, ultrasound. As per the name, ultrasound machines emit a sound- or acoustic vibrations- though the frequency of these vibrations is far above the human ability to hear. These vibrations are introduced to the body with a small, handheld device and subsequently travel through the body’s tissues before being received and analyzed by the machine. The reason the ultrasound can differentiate between your tissue and your fetus’ tissues is because the acoustic vibrations travel at different speeds in different tissues depending on the tissue density- the same way a noise sounds different under water as it does above. The machine can read these differences in wave propagation velocities and transform them, with a bit of math, into an image.
A step up from ultrasound is the x-ray, the first method of medical imagining to be used over a century ago. The x-ray is not a acoustic vibration, but rather belongs to a class of particle/waves which are organized using the electromagnetic spectrum and which range from radio waves to visible light to high-energy gamma rays. The x-ray imagining system is a more rudimentary method, akin to creating an image using a stencil and spray paint. X-rays are quite high-energy particles that are able to pass through flesh (and most other things) quite easily. However, again, the higher density the structure is, the more the x-rays are deflected, especially when it comes to bone. Thus, by placing an x-ray sensitive film on one side of a person’s jaw and an x-ray source on the other, we can develop an image of that person’s teeth. This also works with any other bone in the body.
Though the technique of imaging used by dentists to get a look at impacted molars is fairly basic, this x-ray technology has been ramped up for use in imagining soft tissue as well. Computerized Tomography, commonly known as CT, is a three dimensional version of the x-ray in which the x-ray source and receiver rotate around a patient, gaining full 360 degree perspective if necessary. Using a whole bunch of data processing, computers are able to combine these many perspectives into a 3D image, sort of the way you would walk around statue at the art museum to get a complete idea of its shape. CT allows doctors much more detailed views inside soft tissue, allowing them to tease apart smaller structural maladies such as cancerous growth.
The problem with CT scans, however, is that their source of information, x-rays, are damaging to the tissues they contact. Because they are very high energy, when an x-ray strikes an atom in a DNA molecule within your body, it can alter the structure of that DNA molecule, resulting in a mutation. This is an extremely rare event, but is nonetheless concerning over the course of many x-ray treatments. The final imaging modality I will discuss, MRI, is free from this complication. MRI, short for magnetic resonance imaging, was formerly known as nuclear magnetic resonance, but due to trepidation concerning the word ‘nuclear,’ was renamed. That being said, MRI in no way exposes people to nuclear radiation, in fact it is completely safe. MRI is remarkable for its extremely high spatial resolution and its remarkable adaptability as an imaging tool. MRI can be used to image extremely specific aspects of the human body doing things like showing only blood vessels, or only moving water in the brain, or where fat is located and where water is located. This is largely because the system is so complex that people are able to turn untold number of “knobs” to achieve different results.
The MRI tube in which a patient lies is surrounded by enormous superconducting magnets that operate at many many times the strength of the earth’s magnetic field. The magnetic field results in the alignment of some of the hydrogen atoms within your body’s tissues with the field. These hydrogen atoms possess an inherent spin- the atoms themselves are rotating like a globe- and when they align with the magnetic field, begin to precess around the magnetic field vector like a spinning top. The MRI operator then introduces a radio frequency that serves to displace this precessing atom so that its axis of rotation is changed. However, because the atom wants to be aligned with the magnetic field of the MRI machine, it will return to its original position, which is known as relaxation. As relaxation occurs, the atoms release signals that convey spatial information. These signals are then received by the machine and transformed, again with quite a bit of math, to an image that can be used by doctors.
MRI is constantly being improved upon, and is remarkable in its seemingly boundless opportunity for innovation. It will likely continue to be the dominant form of imaging for years to come.