Passing a rotating fan beam of x-rays through the patient
Basically many x-rays at different angles around the patient
Computer can generate many images from different angles from a single examination
Can even generate a 3D image
10 – 100x more radiation than x-ray
The appearance of tissues on CT
Densities are generally the same as for x-rays
Bright on CT = high density
Dark on CT = low density
The actual radiodensity of a tissue can be measured
The radiodensity of a tissue is expressed in Hounsfield units (HU) or CT numbers
HU is the reduction coefficient of the tissue relative to water
The HU of air is -1000
The HU of fat is -80
The HU of water is 0
The HU of blood is +50
Higher sensitivity than x-ray
Types
High-resolution computed tomography (HRCT)
For lung and skull
Dual energy CT (DECT)
CT scans are presented as a series of slices of tissue
Slices are always viewed as if from below the patient
I.e. structures on the patient’s right side is on the left of the image
Also true for MRi
The slices can be thick or thin
Thin slices (high-resolution CT):
Slower scanning
Increased dose (more pictures are taken)
Higher image detail
Thick slices:
Faster scanning
Lower dose
Worse image detail
Contrast agents
Used in 75% of all CT studies (contrast CT)
Windowing
Windowing is a process where the CT image is manipulated by a computer (the “window” is adjusted)
Adjusting the “window width” adjusts the range of CT numbers which are visible (changes contrast)
Example: A wide window shows tissues with CT numbers between 400 and 2000
Good for differentiating tissues with different radiodensity
Like air and soft tissue
A narrow window shows tissues with CT numbers between 50 and 350
Good for differentiating tissues with similar radiodensity
Like different soft tissues
Adjusting the “window length” or “window center” changes the brightness of the CT image
This manipulation changes the appearence of the picture to highlight certain structures
Bolus tracking
A technique used to optimising the timing of contrast CT
The technician takes a picture before the bolus is injected
Draws a region of interest (ROI) on the pulmonary trunk
The machine will then continously take low resolution CT images through the pulmonary trunk
The contrast bolus is injected
When the Hounsfield unit of the ROI exceeds a certain threshold -> the contrast bolus has reached the pulmonary trunk -> the machine will start to take high resolution CT images of the whole lung
