The CT Scanner

A single plain X-ray image flattens the whole three-dimensional body into one two-dimensional shadow, with every structure along the beam's path superimposed on top of every other. Computed tomography (CT) breaks free of that limitation entirely, reconstructing detailed cross-sectional slices — and ultimately full 3D volumes — from many individual X-ray measurements.

The rotating gantry: tube and detector array

A CT scanner is built around a rotating gantry: a ring-shaped frame that holds an X-ray tube on one side and a corresponding detector array directly opposite it, on the other side of the patient. The patient lies on a table that passes through the centre of this ring, and the tube and detector array rotate together, in fixed alignment with each other, all the way around the patient. As they rotate, the tube fires the beam through a single cross-sectional slice of the patient's body from a huge number of different angles in turn, and the detector array measures the transmitted intensity reaching it at each of those angles.

A narrow, near-monochromatic beam

Unlike the broad, continuous-spectrum beam used in plain radiography, a CT scanner restricts its beam in two important ways. First, the beam is filtered to be near-monochromatic — restricted to a narrow range of photon energies, rather than the full continuous-plus-characteristic spectrum described in the first lesson of this topic. Since different photon energies attenuate differently as they pass through tissue, a broad spread of energies in the beam makes the resulting attenuation data harder to interpret consistently; restricting the beam to a narrow energy range avoids this complication and gives more consistent, reliable attenuation measurements.

Second, the beam is restricted to a narrow width, corresponding to a thin cross-sectional slice of the body at a time. This narrow-beam geometry is what allows CT to build up detailed, thin-slice data rather than a single broad projection — each rotation of the gantry gathers detailed information about one thin slice, and the table then advances so the next slice can be scanned.

The detector array and many-angle data collection

Rather than using one single large detector, as in plain radiography, a CT scanner's detector array is built from a great many individual small detector elements arranged in a row (or a curved arc) opposite the tube. Each individual detector element measures the transmitted intensity along its own narrow path through the patient, for the tube's current angular position. As the gantry rotates through many different angles around the same slice, this process repeats at each angle, generating an enormous set of individual transmission measurements — many detector readings, at many different angles, all referring to the same thin cross-sectional slice.

Computer reconstruction

This large set of transmission measurements, taken from many different angles around the same slice, is passed to a computer for reconstruction. The detailed mathematics of the reconstruction algorithm itself is not required knowledge at this level — what matters is the underlying idea: because each individual measurement reflects the total attenuation along one particular path through the slice, and many different paths from many different angles all cross through the same small region from different directions, combining all of these measurements together makes it possible to calculate the attenuation coefficient at every individual point within the slice. The result is a detailed two-dimensional map of $\mu$ across that cross-section — effectively a full reconstructed image of internal structure through that slice.

Because the patient table advances slightly between each rotation, a CT scan collects data for many adjacent slices in sequence. Stacking these individually reconstructed slices together builds up a full three-dimensional image of the whole scanned region, which can then be viewed as any cross-section, or rendered as a 3D volume.

CT versus plain X-ray imaging

Comparing CT scanning against plain (conventional) X-ray imaging highlights both its major advantages and its real costs.

• Resolution. CT offers dramatically better spatial and contrast resolution than plain radiography. Because a plain X-ray image simply superimposes the whole three-dimensional body onto one flat two-dimensional projection, subtly different soft tissues lying along the same beam path cannot be separated at all — their attenuation differences are all blended together into one combined shadow. CT reconstructs each thin slice independently, avoiding this superposition problem and distinguishing much finer differences in tissue attenuation.
• 3D information. A plain X-ray image is a single flattened projection with no depth information. CT provides genuine cross-sectional and, once slices are stacked, full three-dimensional information about the scanned region.
• Cost. A CT scanner is a far more complex and expensive piece of apparatus than a plain X-ray set, and a scan itself takes considerably longer to acquire and process than a single plain radiographic exposure.
• Dose. A full CT scan requires many individual X-ray exposures, taken from many angles around each of many slices, rather than the single exposure needed for one plain radiograph. As a result, a CT scan delivers a considerably higher radiation dose to the patient than a single plain X-ray image.

Property	Plain X-ray	CT scan
Resolution	Lower; similar tissues along the same path blend together	Much higher; slices avoid superposition of structures
3D information	None — single flattened 2D projection	Full cross-sectional and 3D volume data
Cost	Lower; simpler apparatus, fast	Higher; complex apparatus, longer scan/processing time
Patient dose	Lower; single exposure	Considerably higher; many exposures from many angles

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