COMPONENTS OF A COMPUTED TOMOGRAPHY SCANNER

COMPONENTS OF A COMPUTER TOMOGRAPHY SCANNER

HOW DOES A COMPUTED TOMOGRAPHY SCANNER WORK

A computed tomography scanner is a special type of X-ray machine. It was invented to solve 2 major challenges associated with a conventional X-ray machine

1.       Inability of an X-ray machine to make 3 dimensional images of the objects/ body parts X-rayed. The organs of the body are in 3 dimensions of breadth, length and height, but an x-ray image is in 2dimensions of breadth and height. This makes an X-ray image a limited or false representation of the organs under view.
2.        Superimposition of anatomical structures on each other on X-ray films. For example, when you chest, the ribs lie on top of the lungs. The sternum is superimposed on the thoracic spine, and the right side of the heart lies in between the sternum and thoracic spine and cannot be visualized.

In order to overcome these challenges /limitations, a Computed Tomography scanner works based on 3 basic principles:

1.        That the body is made up of thousands of tissues which different densities (and atomic numbers) and these tissues have different inherent abilities to absorb X-ray photons passing through them owing to their different atomic masses/ numbers. This means that these tissues absorb X-ray photons at different rates to the limit of their natural densities.
2.       . The amount of X-ray photons absorbed by these tissues can be measured and mathematically processed to calculate their attenuation coefficients. These attenuation coefficients form the bases of CT image generation.       
3. The ability to reconstruct 3-Dimensional (cross-sectional) images by systematic computed manipulation.

The process of image generation in computed tomography is divided into 3 which includes

1.       The joint process of passing a uniform X-ray beam through the patient (incident beam) and obtaining a non-uniform X-ray beam that comes out from the patient (emergent beam). Scan phase.
2.        The process of calculating the differences in the amount of X-ray photons that passed through the patient and the amount that came out of the patient. This involves calculating the differential attenuation coefficients the tissues under view. Reconstruction phase.
3.     . The process of  assigning CT numbers to the different attenuation coefficients that represent the different tissues in the body under view. It is the CT numbers that are turned into shades of white and grey that are visible on the screen as CT images. Display phase.

In other to further understand how a CT scanner works, we will discuss these 3 phases separately and appreciate how they are all linked to producing the 3Dimensional CT images.

SCAN PHASE

The scan phase can be acquired in either a step and shoot mode (axial scans) or a continuous motion mode (helical scans). Understanding the difference between a helical scan and an axial scan mode is very important. The gantry is in continuous circular motion during the scan; the table moves patient continuously through the rotating gantry during a helical scan. But during an axial scan, the table moves, then stops and takes a scan, then moves again and stops again and in that order till the scan is complete.

 During a scan (both helical and axial), X-ray is generated by the tube and focused by the collimator system to pass through the body in sections. As the X-ray beam passes through the body, there is differential attenuation by the tissues in the path of the beam. This differential attenuation is characterized by the differences in atomic number and densities of these tissues. The emergent beam carrying data of different linear attenuation of the tissues is recorded, amplified and digitized by a joint action of the detector array and DAS

Let me explain this mechanism in a much simple manner. The human body is made of hundreds of tissues and these tissues have different atomic numbers and masses. Because of this, the tissues absorb X-rays in different proportions. Let’s imagine a particular section of the body is made of up 1000 tissues labeled T_1, T₂, T_3, to T₁₀₀₀. The X-ray beam coming from the tube is labeled X and the X-ray beam coming out from the patient that has now passed through the tissues will be a product of the atomic numbers of the tissues so we will have tissue T₁ giving X₁ and tissue T₁₀₀₀ giving X₁₀₀₀. So what happened is that there are now about 1000 different X-ray energies. When this different energies fall on the detectors, they will make the detector give out different signals each depending on the energy of the energies of the X-rays. So the differential attenuations are used to calculate differential attenuation values by the Data acquisition system. This will be discussed in details later

RECONSTRUCTION PHASE:

The differential (linear) attenuation values recorded by the detectors and converted to electrical signals by the DAS are reconstructed by a complex mathematical algorithm in the CPU. Filtered back projection is the reconstruction programme used in modern day CT scanner. During this process, the image is divided into a matrix of voxels (pixels) and the attenuation coefficients calculated.  The attenuation depends on the density and atomic number of the tissues and energy of the X-ray photons. The objective of CT image reconstruction is to determine how much attenuation of the narrow beam occurred in each voxel of the reconstruction matrix. The calculated attenuation value (coefficients) is what is displayed on the screen

DISPLAY PHASE:

The attenuation values are replaced with CT numbers. A CT number is calculated and assigned to each of the individual tissue voxel of the matrix from the linear attenuation coefficients. The CT numbers are determined by the density of the tissues.

Water is the reference material for CT numbers and has a CT number of Zero. Tissues with attenuation coefficient (density) greater than water have positive CT number; those that are less dense will have negative CT numbers.

CT number is measured in Housfield Unit (HU)

CT number =    U_tissue-U_water   X1000

    U_water

The digital image consisting of matrix of pixels (voxels) with each pixel having a CT number is converted into visible image represented by different shades of gray or brightness level.

Having understood the general overview of how a CT image is formed, we will take a step backwards to discuss the different components of a CT machine and how they function tomorrow.