- The basic plane for CT imaging is -
Transverse
Now, let’s have a look at how the CT image is formed. During the examination, the patient lies in the machine in a horizontal position, so the main plane for us is transverse, or axial plane
- What contrast agent do we use for CT? -
Iodine
Most CT examinations are carried out with the use of a contrast agents as this provides better viewing in order to find a diagnosis. We exclusively use Iodine contrast matter applied either intravenously so we can see the vessels and for being able to eventually evaluate the contrast saturation of certain structures for example tumors; orally to highlight the GI tract or for example in various drains
- By what factor is the Houndsfield unit determined?
By the rate of attenuation of the X-ray beam in the voxel
As with X-ray images, CT is also displayed in shades of grey. Darker objects are hypodense, while whiter objects are hyperdense. On a CT we can express these shades of grey in absolute values – these are called CT numbers, or Houndsfiled units (HU) that express the attenuation of x-ray been in the specific voxel ( kind of a CT pixel)
- Which structure on CT has a density around 0 Houndsfield units? -
Clear fluid
Gas = (-1000) Houndsfield units.
Fat tissue = Approximately (-40) Houndsfield units
Clear fluid = 0 Houndsfield units.
Bone = Approximately 500-1000 Houndsfield units
Metal = Over 2000 Houndsfield units
- Which structure on CT has a density around -1000 Houndsfield units? -
Gas
Gas = (-1000) Houndsfield units.
Fat tissue = Approximately (-40) Houndsfield units
Clear fluid = 0 Houndsfield units.
Bone = Approximately 500-1000 Houndsfield units
Metal = Over 2000 Houndsfield units
- What density does clear fluid have on CT? -
Approximately 0 Houndsfield units
Gas = (-1000) Houndsfield units.
Fat tissue = Approximately (-40) Houndsfield units
Clear fluid = 0 Houndsfield units.
Bone = Approximately 500-1000 Houndsfield units
Metal = Over 2000 Houndsfield units
- What is the density of bone on CT?
Approximately 500-1000 Houndsfield units
Gas = (-1000) Houndsfield units.
Fat tissue = Approximately (-40) Houndsfield units
Clear fluid = 0 Houndsfield units.
Bone = Approximately 500-1000 Houndsfield units
Metal = Over 2000 Houndsfield units
- What density does fat tissue have on CT?
Approximately -40 Houndsfield units
Gas = (-1000) Houndsfield units.
Fat tissue = Approximately (-40) Houndsfield units
Clear fluid = 0 Houndsfield units.
Bone = Approximately 500-1000 Houndsfield units
Metal = Over 2000 Houndsfield units
- What density does gas have on CT?
-1000 Houndsfield units
- What density does metal have on CT? -
Over 2000 Houndsfield units
- Select the best indication for doing a CT examination of the head
– Intracranial bleeding
Head (especially the brain) – Acute problems for this area include trauma, bleeding and vascular obstructions. On this image, you can see subcutaneous hematoma with minor subarachnoid bleeding frontally. CT scans have no place in examining patients with sclerosis multiplex or with epilepsy (MRI examination is best for these patients)
- Select the best indication for doing a CT examination -
Pulmonary embolism
Lungs – CT is main modality for lungs and its pathologies, and we can view acute pathologies very well. Acute pulmonary embolism is the most common pathology visualized by CT. (We have already seen one pulmonary embolism on one of the previous slides).
- Select the best indication for doing a CT examination -
Acute pancreatitis
??Abdomen – CT scan nicely visualize the pathology, but is often unnecessary. Most acute abdominal pathologies can be diagnosed just by clinical examination, blood tests, ultrasound and abdominal x-ray. On this image we can see dilated intestines with air and fluid.
- Radiation dose in a CT scan -
is significant and CT should be indicated cautiously.
Depending on the device used, the investigated area, the specific indication and the method of examination, the dose from one CT scan can vary from approximately 1mSv to 20mSv. Remember that on regular X-ray the dose is approximately 2, 2 mSv. You can see, therefore, that the dose from one CT is high, and it is advisable to approach CT indications carefully. More caution is needed in pregnant women, young women, young children, etc.
