Hello, today we are going to look at a typical CT scanner setup
in a clinical environment. Installing a CT scanner in a hospital requires a lot of preparation.
The floors need to have adequate load-carrying capacity. They also have to be anti-slip and
antistatic due to the presence of sensitive electronic equipment inside the room.
The walls must be constructed with X-ray absorbing material such as barium plaster.
The most noticeable part of the scanner is the gantry with a central opening, into which
the patient is moved during the examination. The detector and the X-Ray tube are located
inside the gantry in opposite side and rotate continuously around the patient.
The X-ray tube is an essential part of the scanner: it produces the X-ray radiations
that pass through the body. The role of the detectors is to measure
the amount of X-ray radiation going through the different organs and convert the radiation
intensities into electrical signals. The electrical signals are then collected
and stored in the computer as scan data.
In some cases, in order to enhance the view of certain organs, the patient will be injected
with a contrast agent also called dye, before the scan.
The dye highlights specific areas inside the body, resulting in clearer pictures. Iodine
and barium are two dyes commonly used in CT scanning.
In the next part of the course, we will introduce contrast agents and how they are used to improve a CT image quality.
The table with the patient can be moved into or out of the gantry as well as up and down.
This allows the patient to be appropriately positioned depending on which part of the
body is being scanned. Here the head will be scanned. A laser is
used to align vertical and horizontal position of the body part.
Before starting a scan, the radiographer will set up the imaging parameters depending on
the area of the body that will be scanned. The voltage and current used by the scanner
will be displayed as well as the windowing parameters W and L.
The radiographer can also check the estimation of the radiation dose delivered to the body part.
In this case, the Computed Tomography Dose
Index indicated as CTDIvol will give a measure of the dose deposited in a single slice of
the patient as mGy. In order to calculate the total absorbed dose
for a complete CT examination, the size of the body part that needs to be scanned must
be taken into account. The dose length product represented here as DLP is the product of
CTDI and the size of the body part. It is measured in mGy per cm.
You will be able to learn more information regarding radiation dose with CT in the next session.
For safety reasons, separate rooms must be provided for the CT scanner and the radiographer.
The rooms must be isolated from each other by a special glass window. What makes it special
is the lead to protect against X-Ray radiation. A computer is used to re-assemble and reconstruct
the data collected by the detectors. By using different mathematical algorithms,
the computer will produce digital images also called 3D volumes.
Then the digital images will be converted into visible images also called analogue images.
The display of this image on the monitor is made possible by using a windowing process.
The radiographer uses software that automatically assigns Hounsfield units values between -1000
to over 1000 to every voxel of a CT image, in order to analyse the data. The Hounsfield
unit is a concept that we will discuss in the next session.
The digital images obtained after reconstructing all the images of the individual slices of
the patient's body are known as tomograms. This set of tomographic images forms the basis
of the next phase of the process: called the diagnosis. During diagnosis, the radiologist
use additional features of the software to identify a region of interest also called
ROI to measure distances on the tomogram, and display the attenuation coefficients of
any of the slices. From the head scan images, three different
views can be accessed, axial, sagittal and coronal views.
The parameters are displayed on the monitor. As you can see, two important parameters are
now displayed. The window width W and the window level L.
Both parameters are used to optimize the
image contrast within the organs to be scanned. This is a 3D image of a heart during a CT angiography.
As you can see the windowing parameters here are set differently than for
the previous head scan.