CT scanner Flashcards
What is an attenuation coefficient?
Measure of how easily a material can be penetrated by an X-ray beam
Give an example of a material with a high attenuation coefficient.
Bone
How does density of a material relate to attenuation coefficient?
The more dense a material the higher the attenuation coefficient.
Summarise briefly the mode of action of a CT scanner
X-ray tube produces X-ray photons which penetrate the patient. Residual beams exit the patient and are detected by the detector array. The detector array can translate the residual beam energy into a measure of attenuation. This data is then transferred to form a digital image.
What is the high voltage generator used for in the gantry?
Supplies a high voltage to the X-ray tube.
Name 4 components found within a CT scanner gantry
1) X-ray tube
2) Detector
3) Collimators
4) High voltage generator
What are individual components of a detector called?
Detector modules
How are the detector modules arranged and what do they make up?
Arranged in rows and they make up the detector array
What does the number of rows of a detector module usually tell us?
How many slices a CT scan produces.
What is the name of the process that happens when a residual X-ray beam hits a detector module?
Scintillation
Explain via scintillation how a digital signal is produced at the detector module?
1) X-ray photos hit the scintilating detectors in the detector modules
2) light photons are produced
3) Photodiodes measure the light produced
4) absorption of light energy creates an electrical signal which is converted to a digital signal.
What is scintillation
This is a process where light is emitted by a material following absorption of ionising radiation.
Name the two types of Collimators found in a CT scanner and what they do
Pre-patient - determine the beam width (usually set to cover the detector array)
Post patient - maintains beam uniformity by absorbing scattered photons
What is the purpose of the BOWTIE filters?
These are found in front of the X-ray tube and ensure that the beam energies are homogenous before hitting the patient.
Name the two types of acquisition model of a CT scan
1) Helical (most commonly used), X-ray beam is ON continuously as patient passes through machine.
2) Axial (step and shoot). Patient is moved through scanner in increments and X-ray beam is turned on to complete a slice and then turned off.
What are two advantages of using a helical acquisition model over an axial one?
Due to the spiral nature of the scan around the body, parts of the body are double dosed which means more data is acquired resulting in a more accurate image.
It is also less time consuming.
Give 4 examples of pre-processing parameters in CT
1) X-ray tube voltage (kV)
2) X-ray tube current (mA)
3)Rotation time (s)
4)Pitch
What is the effect of increasing the voltage supplied to the X-ray tube?
higher voltage = more penetrative X-rays (this can sometimes reduce image quality)
What effect does rotation time have on the patient dose.
A shorter rotation time means less dose to patient. Gantry spins quicker which means exposure to radiation is a shorter time.
What effect does pitch have on the patient dose/image quality
A lower pitch covers more of the patient, delivering more dose but resulting in more data and thus better image quality.
What is pitch expressed as?
Ratio
What is the hounsfield unit?
Relative quantitative measurement of the radiodensity of a material relative to water.
List 3 post-processing parameters
1)slice spacing/interval
2)reconstruction algorithm
3)window width/window level
What two things need to be considered when administering contrast agents?
1) Kidney function
2) Allergies
