Computed tomography
- Fundamentals of Computed Tomography (CT)
- 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
- The radiodensity of a tissue is expressed in Hounsfield units (HU) or CT numbers
- Higher sensitivity than x-ray
- Types
- High-resolution computed tomography (HRCT)
- For lung and skull
- Dual energy CT (DECT)
- High-resolution computed tomography (HRCT)
- 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
- Thin slices (high-resolution CT):
- Slices are always viewed as if from below the patient
- 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
- Example: A wide window shows tissues with CT numbers between 400 and 2000
- Adjusting the “window length” or “window center” changes the brightness of the CT image
- Adjusting the “window width” adjusts the range of CT numbers which are visible (changes contrast)
- This manipulation changes the appearence of the picture to highlight certain structures
- Windowing is a process where the CT image is manipulated by a computer (the “window” is adjusted)
- 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
- Passing a rotating fan beam of x-rays through the patient