# How a Modern CT Scanner Works

How a CT scanner's spinning ring, X\-ray tube and detectors turn many shadow views into cross\-sections and 3D pictures of the body\.

By Di Zhang

Topic: Human Body & Medicine

[Open the interactive Jig](https://ask.jig.com/j/Ms1t0JOUYDgy)

## Transcript

### 1. A ring that sees inside

This is a CT scanner, short for computed tomography\. A patient lies on the table and slides through the ring, called the gantry\. In a few seconds it can make detailed pictures of the inside of the body\. Drag to look around\.

The opening is about 70 centimetres wide, and it is open at both ends\.

### 2. Inside the covers

With the covers made see\-through, the machinery appears\. An X\-ray tube and its high\-voltage generator sit on one side of a big spinning ring\. A curved strip of detectors sits exactly opposite\. Fans and a heat exchanger carry the tube's heat away\.

A slip ring at the back passes power and data across the spinning joint, so the ring never has to stop and unwind\.

### 3. Faster than a heartbeat

During a scan the whole ring spins around the patient, carrying the tube and detectors together\. The patient stays still and feels nothing turning\. A modern scanner can make a full turn in about a quarter of a second\.

That is up to four turns every second, with a heavy X\-ray tube on board\.

### 4. Shadows from every side

The tube sends a fan\-shaped beam of X\-rays through the body to the detectors\. Bone stops many X\-rays, soft tissue stops fewer, and air in the lungs lets most pass\. Each detector measures how much got through\. Turn the tube yourself and watch the shadow change\.

Real X\-rays are invisible\. The glow here only shows where the beam goes\.

### 5. A spiral path

While the ring spins, the table glides slowly through the opening\. Seen from the patient, the tube traces a spiral, like a spring\. This helical scan covers a whole chest in a few seconds\.

Helical means shaped like a helix, the same curve as a corkscrew\.

### 6. From shadows to a slice

Each turn records about a thousand shadow views from different angles\. Smearing them back across the picture gives a blurry slice\. A sharpening filter removes the blur, and a clear cross\-section of the chest appears\. A computer does this in a moment\.

The method is called filtered back projection\. Brighter means the tissue absorbed more X\-rays\.

### 7. Slices into 3D

One scan makes hundreds of thin slices, each less than a millimetre to a few millimetres thick\. Stacked in order, they form a 3D picture of the body\. Doctors can turn it, slice it in any direction, and look at bone or soft tissue\.

Each slice is usually a grid of 512 by 512 tiny squares called pixels\.

### 8. The whole machine, opened up

Here everything works together: the tube and detectors face each other on the rotating gantry, the slip ring and frame hold it steady, and the computer turns millions of measurements into anatomy\. That is how a CT scanner sees inside us\.

Drag to explore the exploded view from any side\.