With modern technology, we can investigate a lot more than what the eyes can see and what the hands can feel. We’ve come to the point where X-rays, ultrasound, CT scans and MRI are available for use for the public for health reasons. In this 2 part series, we will go through roughly what each of these scans show and how they are used to improve your diagnosis and ultimately, managing your health. We will also discuss what these scans do not show, hence the different scans and how they are useful. We will focus on scans that are usually used in physiotherapy management.
X-rays are scans that most people are familiar with. As the name suggests, it uses X-rays which is light of a certain wavelength that passes through the body. The X-rays that passes through then land onto a screen (used to be film) to produce an image. Denser objects such as bone and metal does not allow the X-rays to pass through and appear as white on the screen, while less dense objects such as muscle and water allows X-rays to pass and appear darker/black on the screen. X-ray does use light waves, which means it does have some radiation associated with it.
As the description suggests: it is a 2D image of structures based on density, great for looking at denser objects such as broken bones/fractures, not very useful for looking at less dense objects such as muscle tears. Clinicians, especially radiologists are the experts who can infer based on the position of the bones how the condition of soft tissue might be: e.g. less space between the vertebral bodies/spine may indicate a compressed disc. However, this is mainly done based on the position of the bones rather than being able to see the discs clearly themselves, it may however, indicate that a different scan should be done.
X-rays are cheap and fast to diagnose fractures/bony anomalies and can help to identify other problems, but with only 2D imaging for denser structures, they are not usually conclusive for softer structures (e.g. bursae, discs, tendons, muscles).
Ultrasound are scans that does the complete opposite of X-rays in some ways. It sends sound waves (at a high frequency that you cannot hear) and the reflection of those sound waves are detected and produces an image based on how it is reflected. The sound waves are reflected when there are materials of different acoustic impedances. As it uses sound waves, there is no radiation.
Acoustic impedance behaves slightly differently than X-rays (which is light). Every change in acoustic impedance would cause a slight reflection and produce an image of each layer of changing material (and hence acoustic impedance). This makes ultrasound great for imaging softer structures such as muscles, tendons, ligaments, bursae – but very poor to image very dense structures such as bones and metal as the sound waves will just reflect off them completely. Ultrasound requires a person (sonographer) to operate the machine and takes a longer time as opposed to an X-ray. However, since it does show different layers, it does give a 3D image of the soft tissues being scanned.
Ultrasound is the bread and butter of scanning for softer structures that does not include bones/metal. Great for muscles/tendons/ligaments/bursae and does give a basic 3D image but not in high detail, once again results from an ultrasound may indicate more investigation may be needed.
CT scans or formally known as X-ray Computed Tomography (CT) and sometimes called a CAT scan (Computerised Axial Tomography scan) as it suggests, uses a computer and X-rays to perform the scan. Other forms of computed tomography that doesn’t use an X-ray also exists, such as PET(positron emission tomography) and Single-Photon Emission Computed Tomography(SPECT) scans. However, CT scan usually refers to an X-ray CT, but computed tomography is now used in many different scans.
A CT scan is basically a more advanced “basic” X-ray. It uses a computer to guide multiple X-rays in different angles, producing multiple 2D X-ray images called “slices”. A more sophisticated CT would produce more slices with more detail. Since each slice is a 2D image, and you have multiple slices one after another you would have a 3D image as a result of multiple 2D images. Basically, it uses an older 2D imaging technique to produce a 3D image by doing lots of 2D scanning. Think about a loaf of bread and slicing it very thinly. If you have all the slices, you would be able to see the whole loaf and the surface of each slice, giving you an almost complete 3D image.
The main disadvantages that comes with CT scan compared to the rest does not lie in the imaging itself, but rather logistics. CT machines are costly and bulky, not commonly available in smaller health service centers. This usually means longer waiting times or significantly increased costs depending on your health care system. As mentioned, CT also uses multiple X-rays, which also means increased radiation exposure. Largely because of the cost, CT scans are used after X-rays if X-rays are unable to detect the structural defect or able to detect a possible defect but not with enough clarity.
MRI stands for Magnetic Resonance Imaging. MRI is a medical application of NMR (Nuclear Magnetic Resonance) and used to be called Nuclear Magnetic Resonance Imaging (NMRI). It does not use any radiation unlike CT scan or an X-ray. NMR is a phenomenon where nuclei of atoms would absorb and emit radiofrequencies that can be detected if placed inside a magnetic field. MRI is the medical application of this principle, by placing the body part in question in a strong magnetic field and put sensors to detect the radio frequency emitted by the atoms – at this stage the most commonly used atom to detect is hydrogen. Based on the sensors then computers can map out the image every single hydrogen atom in the scanned body part – yes, every single atom. Unlike a CT, MRI also does not have any ionizing radiation.
If every single atom sounds detailed, that is because it is, it is probably the most detailed imaging technique we have to date. Since we are mainly using hydrogen atoms to be detected right now it would mainly detect fat and water – most of our living tissues are hydrocarbons with hydrogen atoms. Based on the position of these fats and water we can then image almost every structure – e.g. main component of muscle is water. MRI is one of the most advanced imaging techniques we have today and it actually has a clear potential to be even better, right now we are only imaging hydrogen atoms, but the same principle to every single atom in the body which would further improve the image obtained. Even then, just with imaging hydrogen atoms we can paint an almost complete picture of the body, especially soft tissues: muscles, tendons, ligaments.
Why aren’t we using MRI for everything then? There are actually very clear reasons to not use the MRI. MRI is so detailed that for most injuries it is unnecessary. The MRI machine is generally big and bulky. You would have to enter at least the body part of interest into a small tube where the sensors and magnets are – generally uncomfortable for people with claustrophobia or children. The MRI also uses very strong magnets which can be dangerous if you have any magnetic metal implants in your body – these magnets are strong enough to affect objects as heavy as wheelchairs. Always remember to tell your doctor/clinician/radiographer if you have any metal on or in your body. The last and final reason is the same as the CT scan: costs/waiting times. MRI is more logistically heavy even compared to a CT scan. This usually means higher cost or waiting time depending on your health system. This leads to a common approach of using cheaper/faster scans first to determine if you need a more advanced scan, for example:
· Check for bones metals: X-ray first, if needed use a CT scan.
· Check for soft tissues (muscles,tendons, ligaments): ultrasound first, if needed use an MRI.
For clinicians, these scans are classified as “diagnostic tools”, same as your blood tests. Diagnostic tools help clinicians to make a diagnosis, it does not always give the diagnosis directly: if you have a blood test showing you have a low red blood cell count – you have anemia, that is not your complete diagnosis – why you have the anemia is the complete diagnosis, which could be from a number of things such as infection, diet, low iron etc.
In Part 2, we discuss what we might see in scans, but why they might notnecessarily reflect the pain/discomfort you might/might not be feeling. We thentalk about some healing and rehabilitation principles with regards to injury