On the 10th October 2014, our school took our ‘Thank Science It’s Friday’ club on tour to the National History Museum. As part of a research project, I presented a brief insight into the world of medical imaging.
The four mains types of medical imaging used today are X-Ray, PET scan, MRI scan and ultrasound. For each technique I looked into: how they work; the history of the technique; its current medical application and the potential risks, limitations and advantages. My talk prompted a question from the audience about the role of ultrasound in sport, to which I responded with the obvious use of skeletal imaging for fractures. After the presentation I researched further and found that there is also an unproven therapeutic application. Some practitioners harness the thermal effect of the vibrations to try to increase flexibility in muscles, ligaments and tendons, but also to try to increase blood flow to certain areas to reduce healing time.
Below is the information I collated, and on which I based my presentation.
In the year 1892, Sir William Osler wrote the Textbook of Medicine where the scientific community marvelled at new diagnosis techniques such as Haemoglobin estimation, red and white blood cell counts, and simple urinalysis. Today, stimulated by advances in technology, diagnosis has become largely computer based.
Radiology: the use of medical imaging for diagnosis or treatment.
1. X-ray (radiography)
Although X-rays were discovered in 1895, their vast medical application became apparent during the 1960s. A body is exposed to x-rays, which are high energy electromagnetic radiation and the resulting remnant beam is captured. Higher density solids absorb more X-ray photons and through contrasting shades of light, they reveal the internal structure.
The types of X-ray imaging include:
– Projection radiography, where X-rays are mainly used to examine the skeletal system to look for fractures, to take dental x-rays, as a guide for an orthopaedic surgeon in bone replacement and to assist diagnosis of bone cancer. Mammography is a specialised low dose x-ray to detect potentially cancerous tissues in the breast.
– Computed tomography, also known as CT scanning, where high amounts of radiation are used in conjunction with computing algorithms to give a higher definition image.
– Fluoroscopy, where using liquid barium real-time imaging of organ functioning can be observed such as swallowing.
– Dual x-ray absorptiometry or bone densitometry which is primarily used for osteoporosis testing.
It has been proven that too frequent exposure to x-ray radiation can cause cancer. Furthermore, when examining soft tissue, the interpretations can be very challenging. Therefore, they are mainly used as an initial means of diagnosis as there are usually no side effects with occasional use.
2. Ultrasound (sonography)
Ultrasound is used to detect changes in appearance, size, and the contour of organs. A probe called a traducer sends and receives high frequency sound waves which bounce off tissues and the returning pitch and direction produce a real-time image. There are 3 main types of ultrasound: external (through the skin); internal (through the vagina or rectum) and endoscopic (through the mouth or throat). Likewise, a specialised area of ultrasound known as Doppler ultrasound measures the speed of blood cells through vessels.
Ultrasonic energy was first applied to the human body in the US in the 1940s, to assess the thickness of bowel tissue. Nowadays, it is used as a means of diagnosis in organs such as: the Heart (known as an echocardiogram), Liver, Spleen, Pancreas, Kidneys, Bladder and Thyroid. Furthermore, ultrasound has been used to break up gall stones using high frequency waves in the gall bladder. The most ubiquitous use of ultrasound is for monitoring an unborn foetus. Imaging in obstetrics enables a medical professional to determine the sex, location, gestational age, movement and heartbeat, existence of any physical abnormalities in the foetus. In addition, ultrasound is used alongside surgery to guide the needle using real-time imaging.
So far, there have been no known harmful effects, however as waves are intercepted by gas, they are not effective for gas filled organs such as the bowels. Likewise, the waves cannot penetrate dense solids such as large quantities of fat or bones. Whilst there is quite a lot of skill needed to operate and acquire a quality image, ultrasound proves to be the cheapest and most widely available form of imaging which is non-invasive and painless.
3. PET/CT scans (positron emission tomography/computed tomography)
PET systems have been used since 1961 and nowadays over 400 systems have been installed worldwide. Patients take in very small amounts of a radiopharmaceutical which localises in the diseased tissues producing gamma emission. These areas of high metabolic activity in the diseased tissue indicate hotspots that can be represented as a 2D or 3D image of the tissues, which are superimposed on a CT scan. 90% of PET/CT scans worldwide are used to detect cancer, determine how far the cancer has spread and are also useful for studying tissue functioning, e.g. blood flow, oxygen usage and glucose metabolism. They can also be used to study blood flow to the heart muscle and identify whether certain areas would benefit from a coronary artery bypass. Likewise, they are also used to examine renal functioning and the effects of a heart attack. Other uses include evaluating brain abnormalities e.g. tumours, memory disorders and seizures.
PET scanning has a higher radiation risk due to the ionising effect it can have on cells, furthermore, there may be a risk of allergic reaction to the radiotracer. Other disadvantages are that it can be a very time consuming technique, whilst having a lower resolution in comparison with CT and MRI. As with most medical imaging techniques the equipment can be very expensive, although, they do provide unique, accurate and vast quantities of information on functioning. The information from PET scans can provide earlier diagnosis than MRI and CT alone. As with all medical imaging methods they are a less invasive treatment, which so far has produced no long term side effects.
4. MRI scans (magnetic resonance imaging)
To date there are over 25,000 MRI scanners worldwide since the first machine was clinically approved in 1980, at the University of Aberdeen. The technique is based on powerful magnets which are used to polarise hydrogen nuclei in water molecules of tissues. The MRI machine emits a radio frequency pulse and as the protons absorb the energy and spin at a specific frequency, in a specific direction, the energy signals given out by the hydrogen atoms as they try to return to their usual alignment can be imaged. MRIs are used for imaging the physiology of organs and blood vessels, in order to detect tumours, abnormalities, malformation and inflammations. They are an essential component of neuroimaging, and viewing the functioning of systems such as: Cardiovascular (for congenital heart disease); Musculoskeletal (spinal imaging); Liver and gastrointestinal (associated with Cirrhosis and Crohn’s disease) and Oncology (looking at cancerous tissue).
Functional MRI scanning (FMRI) is a technology which has specially been developed for detecting blood flow. When neurons become active, blood moves into the cell, and the levels of oxygen in the blood change. As haemoglobin reacts differently in magnetic fields depending on whether it is bonded to oxygen or not (known as the hemodynamic response), this can be used as an indicator of neural activity. This newer technique plays a large role in research to associate parts of the brain with specific behaviour and can anatomically map the effects of tumours, strokes and neurological diseases such as Alzheimer’s.
As MRI is a relatively new technology there is still uncertainty around the long term effects. Whilst some studies suggest possible genotoxic effects it is deemed safer than x-rays and PET scanning as it doesn’t use ionizing radiation. The machinery is very expensive, loud and confining which can cause discomfort especially with claustrophobic patients. False artefacts can arise due to an irregular heartbeat, breathing and bowel movements. The main advantage is the imaging is not obscured by bone, unlike X-ray, PET/CT and Ultrasound images.
So what is the future for medical imaging?
Even now there seem to be more future possibilities. To briefly outline a few:
Diffuse optical tomography. Using near-infrared light to monitor water and oxygenated or deoxygenated blood which could be extremely effective for detecting tumours.
Elastography. This involves examining the strain and elastin distribution in soft tissues. The underlying theory is that cancerous tumours will be harder than the surrounding tissue.
Electrical impedance tomography. This technique uses electrodes to measure the electrical conductivity of cells. As conductivity depends on the cells free ion content, patterns can be recorded and images developed.