1.1 Introduction to Radiology Basics (CTs and XRs)

Question 1

What are the 4 types of radiological densities?

Answer 1

1. Fluid
2. Air
3. Fat
4. Calcium

Some also cite a 5th, ‘Metallic’.

Question 2

Which imaging modalities constitute ‘conventional radiolography’?

Answer 2

X-rays, also known as ‘plain films’.

Question 3

Name the 2 principles of conventional radiology.

Answer 3

1. Summation of shadows, e.g. in L CXR where you have ‘dense’ appearance due to overlap of structures - heart, vetebrae, mediastinal contents.
2. Silhouette sign, e.g. where you are unable to see the R heart border because it is next to pneumonia of the same density, whereas you can see it in the L heart border.
   
   • In radiology, the silhouette sign refers to the loss of normal borders between thoracic structures. It is usually caused by an intrathoracic radiopaque mass that touches the border of the heart or aorta.

Question 4

What is orthogonal imaging?

Why is it important?

Answer 4

Structures are 3D. Therefore limited utility of 2D imaging modalities.

Orthogonal imaging is a project at 90 degrees original view.

Helps to locate an object seen on an X-ray.

Question 5

What is a CT image composed of?

Answer 5

A matrix made of thousands of pixels, each with a varying Hounsfield unit. It involves ionising radiation.

Question 6

What is a Hounsfeld Unit (HU)?

Answer 6

CT scans use HU to express how dense an object is.

It ranges from -1000 to +1000.

Air is approximately -1000, bone is approximately +400-600.

Question 7

Explain 2 post-processing techniques for a CT scan to highlight additional structures.

Answer 7

1. Changing the window
   
   • Lung window (middle image)
   • Bone window (far right)
2. Changing the plane
   
   • Axial is the normal plane of a CT, it involves looking at the patient from the feet up
   • Coronal is looking at the patient from the front
   • Sagittal is looking at the patient from the side

As these are post-processing techniques, they do not involve additional imaging to the patient.

Question 8

What is IV contrast made of?

What is it used for?

Name 2 major contraindications.

Answer 8

• Low ionic, low osmolar solution with high iodine used to light up vascular structures
• Contraindications:
  
  • Renal failure - can cause acute tubular necrosis
  • Allergy - can cause anaphylaxis

Question 9

What is oral contrast made of?

What is it used for?

How is it administered?

Answer 9

Oral contrast is made of barium sulphate.

It helps to distend and define the bowel.

1000-1500mL is used to define and distend the bowel prior about 60-120 min prior to the examination to allow it to reach the SB and LB.

Question 10

Where does the radiation go during imaging?

Answer 10

Question 11

Which organs are ‘most susceptible’ vs. ‘moderately susceptible’ to ionising radiation exposure?

What is the effect on these organs?

Answer 11

Radiation exposure to these organs can cause increased risk of cancer.

Question 12

Which age demographic is most susceptible to radiation effects and why?

Answer 12

Children - because they have a high number of stem cells, which are very sensitive to radiation.

Question 13

List and describe the 2 major categories of biological damage due to radiation.

Answer 13

1. Deterministic
2. Stochastic

Question 14

Explain the risk of radiation to the unborn child based on weeks gestation.

Answer 14

At 1-10 days, there is greatest risk to the foetus from radiation.

Question 15

Compare and contrast MRI, CT, XR and USS with regards to:

1. Mechanism of acquisition
2. Relative cost
3. Portability
4. Length of examination
5. Contrast

Answer 15