The 50th anniversary of the CT scan

The CT Scan celebrated its 50th birthday in 2021 (NEJM); the first crude scan was taken in 1971. The imaging technique is one of the great technological achievements of the modern era that has impacted every area of medicine. It has become "part of the underlying fabric of routine medical care, producing amazing images that enable accurate diagnoses and efficient therapy for millions of patients each year."

CT is an abbreviation for computed tomagraphy; the latter term refers to "imaging by sections or sectioning through the use of any kind of penetrating wave." In this case X-rays are used to take pictures from multiple angles around an axis to obtain a single two-dimensional (2D) slice of the object (human body). Then, as the object is moved past the X-ray beam a series of slices are taken which can be stacked on top of each other to reconstruct the three-dimensional (3D) object (Figure 1). Over the years more sophisticated instruments coupled with massive gains in computational power have enabled the collection of detailed data that can be transformed into the high-resolution images that we see on the computer screen today.

CT is often confused with MRI (Magnetic Resonance Imaging). Both can take high-resolution three-dimensional (3D) images of the human body but by different modalities. The CT scan uses X-rays as the illuminating beam that can penetrate or scatter off of different body structures before being recorded by a detector. MRI takes advantage of nuclear magnetic resonance in which the body is placed within a strong magnetic field, and then the environment of protons in the body (mainly in water) are probed by exciting at different radiofrequencies (RF). Contrast between different tissues arise from different responses (relaxations) to the RF pulses.

Is one imaging technique better than the other or are they pretty much interchangeable? The answer is that each has its strengths and weaknesses, and both provide high-quality 3D images that contain a wealth of essential medical information.

One difference is that CT (seconds or minutes) is much faster than MRI (tens of minutes) and so is better suited for diagnostic screening. Depending on the tissue, CT can provide higher resolution images which is good for finding fractures in bones or calcification in cornary arteries (coronary CT).

The main disadvantage for CT is that X-rays are a type of ionizing radiation which can damage DNA and cause mutations. Because of the sensitivity of the method, only a small dose is delivered that is essentially harmless, but one must be careful not to accumulate too many doses.

MRI poses no such danger. The magnetic field and radiofrequency pulses are not ionizing radiation and do not harm the body. In terms of imaging capability, MRI is better able to identify certain tumors because of superior contrast to the surrounding normal tissue. But because of its speed, CT is better for scanning the whole body for any cancer metastases.

All in all, the two imaging techniques both overlap and complement one another. They are both remarkable technological achievements that have revolutionized modern medicine. The 50 year anniversary of the first CT scan was truly an occasion to celebrate and in a few years we will celebrate the 50 year anniversary of the MRI.

Figure 1. Images from CT scan of the body. Two-dimensional (2D) scans are reconstructed into a three-dimensional (3D) image. The 3D image can be sliced into 2D sections along any of the three axes so that the interior tissues and organs can be view at high resolution in each plane.