1. How Röntgen Discovered X-rays in 1895
In November 1895, Wilhelm Röntgen in Germany was experimenting with a vacuum tube called a cathode ray tube. He noticed that a nearby fluorescent screen was glowing, even with a wall separating them or thick books placed between the tube and the screen. He confirmed that invisible rays were passing through objects. Because he did not know what they were, he named them X-rays, using the letter "X" to mean "unknown," not his own name [1][2].
Later that same year, on December 22, Röntgen asked his wife Bertha to place her hand on a photographic plate. He exposed it to X-rays for about 15 minutes. The developed photo showed the bones in her hand and the ring on her finger [3]. It is important to note that the day he discovered the rays and the day he took this photo were different days.
3. Why a Single X-ray Photo Makes Body Parts Overlap
X-ray photos show dense parts like bones well, but they have a weakness: everything from the front to the back of the body overlaps in a single image. The shadow of an organ in the front can hide the organ behind it.
Later, CT scans solved this overlap problem. CT can distinguish tissues where the difference in density on the image is less than 1% [5]. To imagine this, think of putting your fingers together in front of a light; the shadows merge into one. However, X-ray photos work differently than light shadows, so this is only an example of the "overlapping" issue.
4. How Hounsfield's CT Measures X-rays from Every Angle
The engineer Godfrey Hounsfield at the British company EMI made CT practical. The starting idea was that "you can know the contents of a box if you measure X-rays from every angle around it" [6]. Separately, the physicist Allan Cormack from South Africa was working on the mathematical foundations. Hounsfield did not know about Cormack's theory at first, so he did not develop everything alone [6].
A CT machine has a donut-shaped part called a gantry. A person lies inside it. The X-ray source rotates around the body, and detectors on the opposite side measure the amount of X-rays received. A computer processes this data mathematically to create cross-sectional images. By stacking thin slices of 1 to 10 mm, a 3D image can also be made [4].
The first clinical scan was performed around 1971 at a hospital in London for a woman suspected of having a brain disease. Because sources differ on whether the year was 1971 or 1972, and on the specific diagnosis, we refer to it as "around 1971" [6][7]. Early images were 80x80 pixels, and it took about 35 minutes to take 6 slices. Later, scan times shortened to 1 to 10 seconds [7].
5. The 1979 Nobel Prize for CT and How CT Came to Japan
In 1979, Hounsfield and Cormack shared the Nobel Prize in Physiology or Medicine for "the development of computer-assisted tomography" [8].
In Japan, one source states that the first CT scanner in the country was introduced at Tokyo Women's Medical University on August 26, 1975. Toshiba developed the first whole-body CT scanner made in Japan in 1978 [9]. However, other records say head CTs were introduced in 1973. Because the years vary by source, this article presents the information from one specific source.
6. How MRI Uses Magnets and Radio Waves to See Soft Parts
MRI uses a strong magnetic field to align the hydrogen nuclei (found in water) inside the body. Then, radio waves are applied. When the hydrogen returns to its original state, it emits signals that are collected to make an image. Unlike X-rays, MRI does not use ionizing radiation, which can damage cells [10].
MRI shows soft tissues like the brain, spinal cord, nerves, muscles, tendons, and ligaments more clearly than X-rays or CT. Because there is no radiation exposure, it may be selected when repeated scans are needed, such as for the brain or children. However, the strong magnetic field can attract iron or magnetizable metals. Therefore, if you have iron or magnetizable metal in your body, you must report it before the exam [10].
7. How Lauterbur Made the First MRI Image of a Beaker of Water
In 1973, Paul Lauterbur in the United States published a method for creating 2D images by adding a gradient (tilt) to the magnetic field in the journal *Nature*. One of the earliest images taken was not of a human body. It was a beaker of water with two test tubes containing heavy water inside. The first submission was reportedly rejected because the image was unclear [11].
Peter Mansfield in the UK developed an efficient method to create images in seconds by mathematically analyzing how to capture signals using magnetic field gradients. In 2003, Lauterbur and Mansfield received the Nobel Prize in Physiology or Medicine for "their discoveries concerning magnetic resonance imaging." According to the award materials, more than 60 million MRI exams were performed worldwide each year at that time [12]. This number refers to around 2003, not the current count.
8. How X-rays, CT, and MRI Compare
You can try comparing these three technologies in a notebook. Make a table with three columns for X-ray, CT, and MRI. Write in your own words: "What does it use?", "What does it show well?", and "How does it handle overlapping?"
You can also look at information from a nearby hospital. Check their website to see what imaging machines they have. Japan is among OECD countries with high numbers of CT, MRI, and PET machines per person. According to OECD data, there are 184 machines per million people in Japan, compared to an OECD average of 51 [13]. Note that this number is the total for all three types, not just CT or MRI alone.