Modern Physics and Medical Diagnostics

Physicists Science and technology are partners - the joint venture between which has brought the world to where it is now. The industrial revolution in the 18th century led to economic growth and scientific discoveries and inventions. Physics and Physician both words have similar meanings - related to natural science. If it is about understanding nature - it is physics and if it is about understanding the medical condition of a human being - he is a physician.

Physicists have been exploring regions beyond visible light for 200 years. The curiosity of Sir William Herschel realized the existence of Infrared light. Ritter pursued the purple end and discovered ultraviolet light. James Clerk Maxwell in 1967 proposed long wavelengths. The discovery of x-rays and gamma rays brought us closer to the reality that all lights are a part of a big picture called the electromagnetic spectrum. As shown in the image below the wavelengths range from a millionth of millimeter to thousand meters to make a wave. Differing wavelengths have different properties which can be utilized in so many different ways. This image below summarizes the gist of all physical properties used in medical diagnosis.

Figure 1: Electromagnetic spectrum with properties and applicability of different wavelengths. 

(image taken from the book, The Essential Physics of Medical Imaging, Wolters Kluwer, 2011)

Scientists realized the existence of the electromagnetic spectrum and started exploring various wavelength regions. Human eyes can see only a portion of light but the enormous variety of properties that the spectrum brings is amazing. Higher wavelengths are used for communications devices like satellite, mobile phones. As wavelengths get shorter they can penetrate and ionize the medium. As the frequency increases in the above image, we know from Planck's hypothesis that the energy of an individual photon increases. And the penetrating power of individual photons of energy increases. The radiation then gets capable of interacting with miniscule particles like DNA and the nucleus of an atom. The possibility of these interactions has opened up innumerable applications in the medical field. Be it radiography, fluoroscopy, mammography, computed tomography, or magnetic resonance imaging. The gates of possibilities just opened up. The advances in physics made the medical field even more capable of diagnosing, treating medical complications. 

The Sun, our star, emits almost the whole of the spectrum. We know that the total emission is according to Stephen Boltzmann’s Law and the wavelength at which emission is maximum is as per Wien's Law. The emission travels through so many layers like corona (sun’s atmosphere), interplanetary space, earth’s magnetic field, our atmosphere, and its different layers and finally reaches the surface.

The wave theory of light formulated by the Dutch mathematician-astronomer Christiaan Huygens has opened a new field of electromagnetics. The classical theory of electromagnetism further emboldened the theory and opened up a vast area of applications - one of which was in the medical field. 

Consider old medical treatments when major physics inventions were yet to be incorporated into the diagnosis and treatment of the human body. Elephant bile was used to treat bad breath, Python bile was used to treat female genital ulcers, Snails were used to treat warts, hemorrhoids were treated with a hot iron, Heroin cough syrup for kids was used to be given, Brush teeth with urine to make it shiny white. Cocaine powder was given to patients suffering from depression - unbelievable practices. Aren’t they? Doctors - not sure they would be even called doctors - had no other knowledge even if the disease is known - the physics was yet to discover the Electromagnetic spectrum. The physics was yet to know about frequency and photons associated with energy. And as one would read below, one would realize how physics has revolutionized our diagnosing capabilities and contributed to improve our life.

Medical Diagnostic Tools:

1. X-ray and CT Scan

In 1895, Roentgen accidentally discovered x-rays. X-rays are light like electromagnetic radiation that has a very short wavelength and high frequency with a high energy level that can penetrate material and break chemical bonds. Researchers and the public got so much interested in these mysterious rays. The rays have an amazing capability of passing through solid matter or the human body. By keeping photographic plates behind one can obtain images or information about the internal structure of the matter. Roentgen imaged the needle in the patient's hand using this idea. Much of the technique remains the same however the evolution to obtain 3D images has increased the efficacy of x-rays in medical imaging diagnosis.

Figure 2: First x-ray of a hand taken by Roentgen.

 (Image copied from wikipedia.org)

Figure 3: Computed Tomography Scanner 

(Image taken from wikipedia.org)

The electrical music industries (EMI) and their recordings with the Beatles played a major role in the research funding of Dr. Godfrey Hounsfield for exploring the use of differential x-ray attenuation to produce back-projection images during the 1960s. The attenuation enabled to compute the image of the target. The discovery was considered to be the greatest advance in radiology medicine similar to the discovery of x-rays. Computed Tomographic Scans looked like magic, as it reconstructs the 3D image of an internal body part. It was amazing and revolutionary. And its evolution from first-generation scanners which created 80 x 80 layers in 5 minutes to now 512 x 512 layers in 1 second is an increase in efficiency of 1.5 billion times. 

CT Scan, a radiological imaging technique, scans thin slices with a rotating x-ray beam. The output is a measurement of x-ray attenuation coefficients along the beam path. Images thus taken from different angles are combined using reconstruction algorithms to visualize bones, soft tissues, and blood vessels.

2. Ultrasound

Ultrasound refers to high-frequency sound above 20 KHz which is beyond human listening capability. The ultrasound is a mechanical wave that can propagate in the matter but some of the body parts like tissues reflect in proportion to the density of the material. By studying images produced by echo of an ultrasonic sound keeping in mind the physics of interactions between different body parts and ultrasound, a two dimensional image is constructed and a conclusion about the state of the body part is obtained, popularly known as sonography. Transducer probe is the main part of the machine which produces sound waves using piezoelectric effect. It also receives echo. This echo is converted into a two dimensional image using a computer algorithm. 3D ultrasound imaging uses the doppler effect and gives a better look at the organ which is examined. 

Figure 4: Ultrasound machine with various transducer probes. 

(Photo courtesy Dynamic Imaging Limited)

The most advantageous feature of ultrasound is it doesn’t require any radiation. It is useful in determining the size of the fetus, to see the possible abnormal structure inside the heart, to identify kidney stones. The ultrasound is more beneficial when we want to see muscle or soft tissue. 

(c) Magnetic Resonance Imaging

Another Nobel prize winning physics discovery - nuclear magnetic resonance. Consider a nucleus spinning around its axis placed in a strong magnetic field. The nucleus rotations per second is called Larmor frequency. Placing this nucleus in an external magnetic field, energy gets absorbed and it goes from higher to lower energy level. When the external magnetic field is switched off, the nucleus will resonate at characteristic frequency. The protons inside our body are placed in a magnetic field and a radiofrequency current is passed through the patient, the protons get stimulated. As soon as the radiofrequency field is turned off protons release energy which is detected by sensors. The amount of energy released and time taken for protons to re-align depends on molecules inside our body and we are enabled to identify different tissues, bones, and their structure.

Figure 5: Magnetic Resonance Imaging Scan of brain

(image taken from visualonline.cancer.gov)

The use of no harmful radiation makes this non-invasive imaging technology. This imaging technique is used to identify issues with soft tissues like muscles and ligaments. It is also used to diagnose problems in the brain like tumors. Another area of application is a little improved technique called functional magnetic resonance imaging to determine active areas of the brain during cognitive tasks besides helping us in understanding how the brain works. The difference between x-rays and MRI is that x-rays help us identify broken bones whereas MRI scans enable us to see soft tissues, ligaments and circulatory systems. 

(d) Lasers in Medicine

LASER has been one of the most exciting discoveries. The idea of pumping and getting atoms together in an excited state and then making them emit similar photons coherent in nature and moving in the same direction with the same frequency. This property of photons based on the principle of stimulated emission and moving in tandem can reshape corneas, can melt stones inside the gallbladder, clear arteries, and burn tumors - tremendous applications not only in the medical field but in engineering as well.

Most laser surgery is based on laser ablation. As 70-90% of human tissues is water, it can be vaporized using a pulsed laser. The possibility of thermal damage is minimized by having a pulse duration less than thermal relaxation time. 

Diseases in which these tools are used:

The benefits of using radiation for imaging carries a certain risk. Two kinds of effects are possible. One is the effect on body parts which is called Somatic effects whereas the other is Genetic effects. While taking an X-ray or doing a CT scan, a part of x-ray gets absorbed. This absorbed radiation is known to induce cancer growth of tissues and possible genetic disorder. Prolonged high doses of x-rays may result in adverse health effects. Using ultrasound waves on body parts includes heat absorption by tissues which contain a lot of water. This increases the temperature locally and may cause tissue damage. Tissue cavity is seen to be formed at the place where the head is deposited.

Figure 6: Skin disease due to radiation exposure
(image taken from www.healthline.com)

The cells that may be killed due to heating radiation are detectable but long term effects like tissue breakdown or scars take years to be seen. The skin is affected in various ways. Temporary reddening of skin of exposed body parts is a first sign of side effects. Prolonged exposure blisters and skin ulcers are developed, loss of hairs and skin color change is evident. Inside the cell chromosome and genes might be damaged.  Cell killing due to radiation inside the gastro tract may cause diseases like dysentery. Similarly the other effects on body parts include effects on reproductive organs, eye lens damage, radiation sickness and brain cells damage. We all have bone marrow cells. This makes cells that form blood. The radiation affects these cells the most. Depending upon the dose of the radiation, the effects on blood cells may result in infection or hemorrhage. The fast changing fields in MRI gives a twitching sensation and nerves get stimulated.

Safety measures:

To safeguard against harmful radiation during medical diagnosis the essential steps include. (1) safety of patients from harmful radiation (2) image quality; (3) quality assurance and (4) compliance with regulatory agencies.

Figure 7: A typical radiation safety suit to prevent harmful exposure

(taken from wiki commons)

The most effective protection from radiation is to control the amount or dosage. The radiation dosage is measured in millisieverts and based on the recommendation by experts the radiation is determined. Besides certain rules are prescribed while performing some of the tests for example while taking magnetic resonance imaging people implanted with stimulators or pacemakers are not allowed. To protect one from noise generated during MRI ear protection gear are used. As a precautionary measure MRI scans are not recommended during pregnancy especially when fetus organs are formed.

Thus, it may be concluded that beginning from the early 20th century, the inventions of physics has led to enormous growth of using tools in exploring human anatomy and helped us in medical diagnosis. It helped us in identifying disease in full detail. It enables us to see bones, tissues, muscles, brain, circulatory systems, everything that was not possible earlier.