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Medical imaging is the process of creating visual images of the body’s internal structures normally hidden by the skin and bones in order to diagnose and treat disease.
Medical imaging began with radiography, following the discovery of X-rays by Wilhelm Röntgen in 1895. It was observed that the radiation from X-rays is absorbed in differing amounts by the various tissues it penetrates, depending on the density of the tissue, and when this is produced on a photographic film a distinguishable image is created.
The main limiting factors of X-rays are the harmful effects of radiation, which were not fully realised at the time of its discovery, and how similarity in densities of adjacent tissues in the body makes X-rays useful for imaging bone or foreign objects, but not for soft tissue pathologies.
While the purpose of medical imaging has remained the same – to study the body’s internal workings for clinical analysis – the types of energy used to create medical images have evolved over the years.
X-rays remain the most common form of imaging, used regularly to check bones for breaks and lungs for shadows that could indicate pneumonia or cancer.
X-rays were once stored on hard film copy, but they are now mostly used in digital formats. Originally only 2-dimensional images could be captured, however Computed Tomography (where the tube and detector both rotate around the body) now allows for full 3-D visualisation.
The most common forms of X-rays include radiography, Computed Tomography, mammography (breast screening for cancer), angiography (scanning of blood vessels and organs) and fluoroscopy (creates a continuous ‘live’ image on a monitor).
Computed Tomography is an advanced form of X-ray that allows the body to be examined in cross sections, like the slices in a loaf of bread. It is used to examine the brain, spine, chest, abdomen, pelvis and limbs, and employs a large circular scanner through which the patient passes on a mobile table.
CT scans can produce images of every kind of body structure, including organs, bones and blood vessels, but a contrast material is sometimes required (injected or swallowed) to improve picture quality. Some soft tissue details are still hardly visible, and require the use of Magnetic Resonance Imaging (MRI) instead. Another drawback of Computed Tomography is it produces more radiation than a normal X-ray, so it isn’t used regularly.
Ultrasound uses high frequency sound waves which are absorbed as heat by the body. Rather than the still images produced by X-rays, a hand held ultrasound transducer converts the sound waves into electrical impulses to display ‘real time’ moving pictures on a screen.
Ultrasound is a useful diagnostic tool for measuring foetal size in pregnancies, and for examining the heart, blood vessels, muscles, joints and pelvic organs. It is much safer than X-rays because there is no radiation, and is useful in producing highly detailed images of the smallest parts of the body.
Magnetic Resonance Imaging uses a magnetic field and radio waves to create high-resolution 3D images of the body’s organs and soft tissues. It is often used for brain and organ scans, and also to examine injuries in joints and ligaments.
Like CT scans, a contrast medium may be required for greater clarity. But, unlike CT scans, MRIs do not involve the use of ionising radiation. MRIs produce superior image quality, but are a long and noisy procedure (the machine is very loud), and any slight movements can ruin the image, making them a last resort for small children or sufferers of claustrophobia.
Positron Emission Tomography is a type of nuclear imaging where a radioactive tracer is inhaled, swallowed or injected into the patient. This tracer emits gamma-rays which the PET scanner uses to create images on a screen. These tracers, known as radiopharmaceuticals, attach themselves to specific cells and their distribution within the body can indicate how well organs and tissues are functioning.
The main advantage of PET over most other forms of imaging is that it can detect problems much earlier than other techniques. It is often used in cancer treatments, as it can indicate how far a cancer has spread and how well the treatment is working.
One of the biggest advances in medical imaging has been the move from photographic film to digital images. Digital images can be stored and retrieved quickly and easily and – especially thanks to cloud storage – they can be shared all the world, greatly improving access to expert diagnosis.
Digital imaging is also contributing to much faster and more accurate diagnosis, with machines being programmed to read large amounts of data at high speed, and to refine content for human analysis to only those images that are of highest relevance.
Future medical imaging will also see the problem of radiation reduced to negligible levels, with new imaging techniques like Phase Contrast X-ray Imaging (offering much higher contrast) having the potential to reduce radiation doses by 10 to 100x fold or more.
The way biological tissues are studied using different forms of energy is changing continuously and at a relentless pace. Medical imaging is moving beyond just anatomy and pathology, to physiology, pharmacology, and cellular and molecular biology. And such is the pace of change, it is no longer fanciful to imagine that one day we will have the tools to be able to map the internal human body in minute detail, examine its workings in real time, and identify and rectify problems as and when they arise.