I am often asked by patients to explain the difference between an MRI scan and a CT scan. Others presume that the most expensive kind of diagnostic imaging must be inherently better, so why settle for a relatively cheap ultrasound exam rather than the more advanced and costly MRI or CT options? I usually reply by comparing the options available to a household toolbox: the hammer is best for knocking in nails, but no use at removing screws. Similarly, ultrasound can’t detect problems in the spine and MRI won’t spot the early stages of lung cancer.
Thanks to technological advances and more reliance on imaging to diagnose and monitor disease, there has been a huge growth in the number and range of scans. Both the public and doctors now face a potentially baffling array of tools, so the role of the clinical radiologist has become more important. He or she is ultimately responsible for the interpretation of scans and these days increasingly involved in decisions about clinical management and treatment.
Lately much has been said about improving access to ‘cancer scans’ to aid early diagnosis. The first question is: which type of scan? MRI (Magnetic Resonance Imaging)? CT (Computed Tomography)? Nuclear medicine, ultrasound, perhaps PET (Positron Emission Tomography)? As a consultant radiologist with first-hand experience of the proliferation of methods, I want to shed some light on this complex area. So here’s my rough-and-ready guide.
This technique transmits high-frequency sound waves of 20,000 or more vibrations per second — above the range of human hearing — into a patient’s body, then analyses the reflections. It is the same technology as the military sonar used to detect submarines, and was developed in the late 1940s by Dr George Ludwig at the Naval Medical Research Institute in Maryland. It came into medical use thanks to work by Professor Ian Donald in Edinburgh, who developed the practical technology of obstetric ultrasound in the 1950s.
It works by using reflected sound waves to construct an internal picture of body tissue, and a video or still image record can be kept. It is comparatively cheap, widely available, and there are no known harmful effects at the low energies used in routine diagnostics. This makes it especially useful for antenatal and paediatric imaging. Higher-frequency probes provide clearer resolution but cannot ‘see’ to the same depth as lower frequencies — the operator will decide which is best. Increasingly, as the machines become smaller and cheaper, ultrasound will be used like a modern stethoscope.
Echocardiograms use ultrasound and the Doppler effect to assess blood flow, especially in the heart. Diagnostic ultrasound differs from the higher-energy kind used in physiotherapy, which is thought to stimulate circulation and cell activity. This is also used to treat a variety of conditions with a technique known as HIFU (High-intensity Focused Ultrasound), which heats tissues to make them die and shrink.
Computed Tomography (CT)
This was developed by British electrical engineer Sir Godfrey Hounsfield while working for EMI in the 1950s and ’60s, and won him a Nobel prize. Though first known as an EMI scan, contrary to urban myth it was not associated with the profits from sales of Beatles records!
Tomography describes the process of scanning in ‘slices’ to construct a cross-sectional image. CT does it with penetrating X-rays and computers. Modern scanners can generate high–quality images at much lower levels of radiation than the original machines. In a single heartbeat, with X-ray intensities of less than 1 milliSievert (about three to four months of background radiation in parts of the UK) a 3D image of the heart and coronary arteries can be obtained. There is little, if any, evidence of harm. For a one-year-old child the theoretical lifetime risk of developing cancer from an abdominal CT is about one in 1,000. This risk decreases with age and compares to an average real lifetime cancer risk of one in three.
CT is now replacing invasive procedures such as angio-graphy and colonoscopy, and is used to diagnose and monitor cases of cancer, stroke and multiple trauma. Low-dose CT can also screen patients with no symptoms for colon and lung cancer, as well as identify people at risk of heart disease and stroke.
Magnetic Resonance Imaging (MRI)
Powerful magnetic fields and radio waves work with computer algorithms to generate images, in this case by recording the resulting behaviour of protons in water inside the body. This technique also has deep roots in the UK — a lot of pioneering work was carried out at the University of Nottingham in the mid-1970s by Sir Peter Mansfield (another Nobel prize winner). The first MRI pictures had a low resolution and took a long time to produce, whereas today’s are quicker and can show very fine detail. And Functional MRI can show processes such as brain activity. These changes have come about thanks to a combination of a massive improvement in scanner hardware and computing power, plus better techniques.
MRI is good at investigating neurological and musculoskeletal problems. And since it does not use X-rays and causes no known harm, it can be seen as better for children. The downside is that examinations take longer, sometimes up to an hour, and a minority of people cannot tolerate being in the enclosed scanner for that long.
Certain precautions are needed before a patient goes in. Those with any ferromagnetic material in their body — from surgery, previous injury or even some tattoo pigments — may be at risk. And most patients fitted with a heart pacemaker cannot have MRI scans.
Radioactive isotopes are injected into the patient then absorbed, to varying degrees, by organs in the body. The concentration or extent to which they are taken up is then used to create images based on radioactive decay, detected by a device called a gamma camera. The most commonly used compound is called Technetium 99m and it is used in bone scans — commonly to search for the spread of cancer or infection. Thallium is used in cardiac scans, and iodine to evaluate thyroid disease. Nuclear medicine is better than other scans in showing the function of the target rather than simply its anatomy.
Varieties called PET (Positron Emission Tomography and SPECT (Single Photon Emission Tomography) use isotopes to construct image slices in a similar way to CT, but based on function. These images can be overlaid on higher quality anatomical images from CT scans, resulting in a combined image known as PET-CT. Those used in combination with MRI are called PET-MR.
More commonly known as dye and used to improve the contrast between different types of tissue during scan. The substances involved are safe for most patients. There is a very small risk of a serious allergic reaction for just one in 40,000. X-ray contrast agents are usually iodine-based but any allergic reaction is to other compounds within the agent. Because these substances are mildly toxic to the kidneys, it is important to know about any existing renal disease as well as conditions such as diabetes.
This has become a hot topic in public health and radiology circles, but while high doses of radiation have been clearly shown to increase the risk of future cancer, the risk from medical diagnostic radiation is much harder to demonstrate. Any harmful effects are too small to measure, and are currently estimated on the assumption that if very high doses cause cancer and a zero dose is safe, then the risk in between is shown by a straight line on a graph. Many studies suggest that this is not the case. Alternative theories even suggest that low doses of radiation, equivalent to ten CT scans a year, result in increased lifespan. So while it’s sensible to limit the amount of diagnostic tests, perhaps the more important reason is to ensure the facilities are available to as many patients as possible.
Dr John Giles is a consultant radiologist and medical director of the Beneden Hospital Trust.