Imaging Methods: X-ray, CT and MRI Explained
An X-ray image is a two dimensional map of how much X-ray energy passed through your body and reached a detector. Dense tissue such as bone absorbs more of the beam, so less energy arrives there and that area looks white; air absorbs almost nothing, so it looks black. That is why the result is often described as a shadow: the body casts a pattern onto the detector, and we read the pattern rather than the body itself. A CT scanner takes that same idea and turns it into a computed slice by measuring the beam from many angles and letting a computer reconstruct a cross section. An MRI scan uses no ionising radiation at all, because it works with a strong magnetic field and radio waves instead of X-rays.
What is an X-ray image and why is it like a shadow?
Think of standing between a lamp and a wall. Your body blocks some of the light and the wall shows a dark outline of you. An X-ray room works the same way, except the lamp is an X-ray tube, the wall is a detector, and the outline is produced by how much each tissue absorbs. Bone, being dense and full of calcium, stops a lot of the beam and appears white. Soft tissue stops some of it and appears in shades of grey. Air in the lungs or bowel stops almost none of it and appears black. Metal, such as a hip replacement or a surgical clip, stops nearly all of it and appears bright white.
The word shadow is useful but it can mislead. A shadow on a wall is a silhouette, and so is a radiograph: everything along the path of the beam is superimposed on one flat picture. A rib and a lung lesion at different depths can land on the same spot. That is the main limitation of plain radiography, and it is exactly the problem that tomography was invented to solve. If you want a patient level walkthrough of how the request, the appointment and the report fit together, this X-ray image, shadow explanation covers the same ground in plain English.
Two practical points follow from the physics. First, the technologist positions you carefully because the angle changes what overlaps what. A chest X-ray taken straight on shows the heart and lungs differently from one taken from the side. Second, the amount of radiation is kept as low as reasonably practicable, which is a legal duty in the UK under the Ionising Radiation (Medical Exposure) Regulations 2017, usually shortened to IR(ME)R 2017. The regulations place duties on the referrer, the practitioner and the operator, not only on the radiographer who presses the button.
How does a CT scanner create a computed slice?
A CT scanner, short for computed tomography, takes the shadow idea and rotates it. The X-ray tube and the detectors sit opposite each other on a ring and spin around you while the table moves through the opening. Instead of one projection, the system collects hundreds or thousands of projections from different angles. A computer then solves the reconstruction problem: given all those overlapping shadows, what distribution of tissue would produce them? The answer is a cross sectional slice, and the process is why the word computed is in the name. The mathematics behind it was worked out by Godfrey Hounsfield and Allan Cormack, who shared the 1979 Nobel Prize in Physiology or Medicine for it.
Each pixel in the slice is given a number on the Hounsfield scale, which sets water at zero. Air is around minus 1000, fat is negative, soft tissue sits near zero to plus 60, and dense bone runs into the hundreds or over a thousand. Because the numbers are quantitative, a radiologist can measure a nodule, compare it with a previous scan, and judge whether it has changed. That is much harder to do from a single flat radiograph.
CT does involve ionising radiation, and doses are higher than for a single plain film because of the number of projections. This is where justification and optimisation matter. Justification means the examination must be worth doing for this patient, with the benefit weighed against the risk. Optimisation means using the lowest dose that still gives a diagnostic image. Modern scanners adjust the tube current as they rotate, so thinner parts of the body receive less. Contrast agents, given by mouth or into a vein, are sometimes used to make blood vessels or bowel stand out, and they carry their own checks and consent conversation.
Why does an MRI scan not use ionising radiation?
MRI stands for magnetic resonance imaging, and nothing in the process involves X-rays. The scanner produces a very strong static magnetic field, typically 1.5 or 3 tesla in clinical use, which aligns the hydrogen nuclei in your body. A radio frequency pulse then tips those nuclei out of alignment, and as they relax back they emit a faint signal that coils around you detect. The computer turns that signal into an image. Radio waves and a magnetic field are not ionising, meaning they do not have enough energy to strip electrons from atoms and damage DNA directly, which is the mechanism of concern with X-rays and CT.
That does not make MRI risk free. The magnetic field is always on, and it can pull ferromagnetic objects with force. Anyone with a pacemaker, cochlear implant, aneurysm clip, metal fragment in the eye or certain other devices needs to be screened before entering the scanner room. Loose oxygen cylinders, scissors and keys have caused serious accidents. The safety questionnaire is not bureaucracy; it is the main protection. Some patients also find the noise and the enclosed space difficult, and the scan takes longer than a CT, often 20 to 45 minutes per area.
MRI is particularly good for soft tissue: brain, spinal cord, joints, pelvis and liver. CT remains faster and better for bone detail, acute bleeding and lung imaging. Ultrasound, the fourth common method, uses high frequency sound waves and their echoes, involves no ionising radiation either, and is often the first choice for the abdomen, the heart and pregnancy.
Who actually performs and reports these examinations?
A radiologist is a doctor who has completed specialist training in interpreting images and who reports the findings. A radiographer, called a radiologic technologist in some countries, operates the X-ray, CT and MRI equipment and positions the patient. A sonographer performs ultrasound. In the UK the request usually starts with a written referral from a clinician, which is then checked by the imaging department against local rules before it is accepted. The report comes back to the referrer, not directly to the patient in most pathways, which is why results can take a few days to reach you.
Doses are quoted with a source and a year because they change as equipment and protocols change. A chest X-ray is a small fraction of natural background radiation; a CT of the abdomen is much larger, equivalent to a few years of background in some estimates. These figures are useful for perspective but they are not a reason to refuse a scan your clinician has justified. The right question is not whether there is any radiation, but whether the examination is justified and optimised for your situation.
What should you take away before your appointment?
Know which method you are having, because the preparation differs. CT with contrast may require blood tests for kidney function and a period of fasting. MRI requires the safety questionnaire and the removal of all metal. Ultrasound usually needs a full bladder for pelvic scans and no preparation for others. Arrive early, bring your referral details and a list of implants, and tell the staff if you are or might be pregnant. If you are unsure why a particular examination was chosen, ask the referrer. A short conversation about justification is part of good care, and it is written into the regulations that govern imaging in the UK.