Lecture One: X-ray, CT, PET

Question 1

What are the four main techniques of functional and anatomical brain imaging?

Answer 1

Radiography and X-ray angiography
CT
PET
MRI

Question 2

Describe the physical basis of x-ray;

Answer 2

Absorption of x-ray by tissues

Question 3

Describe the x-ray procedure;

Answer 3

• Broad beam of x-rays pass through head onto x-ray film

- Results in projection (2D, shadow) image of the 3D object

Question 4

How does the X-ray image relate to structure;

Answer 4

• Intensity is in proportion to absorption pf x-rays

i. e white = dense

Question 5

How do angiograms, a form of x-ray differ from usual?

Answer 5

They use a contrast agent typical iodine which is injected intravascularly and it has a high attenuation co-efficient.

Question 6

What does attenuation describe in terms of x-rays?

Answer 6

How well it is penetrated by light or in this instance x-rays

Therefore bone has high attenuation but soft tissue does not.

Question 7

What are the uses of x-rays?

Answer 7

• Skull fractures, bone abnormalities (bone tumours)

- Symmetry of structures can be observed

Question 8

What do angiograms show?

Answer 8

Angiograms show aneurisms, vascular defects i.e occlusive strokes

Question 9

What is the advantages of plain film x-ray?

Answer 9

• Very high spatial resolution (0.05mm)

- Good depiction of the skull (fractures tumours)

Question 10

What are the disadvantages of x-rays?

Answer 10

• Projection images only
• Poor intrinsic tissue contrast
• Use of ionising radiation
• Some risk associated with injection of contrast

Question 11

What is CT?

Answer 11

The use of X-ray imaging but in many planes to create a 3D image and then create slices.

Question 12

Whats the physical basis of CT?

Answer 12

The absorption of x-rays by tissue

Question 13

Describe the procedure of CT;

Answer 13

• Narrow x-ray beams are projected through the head onto detectors
• The source is rotated about the head to aquire many projections
• A 2D image of the slice is reconstructed using filtered back projections
• An adjacent slice is imaged building up a 3D image

Question 14

What are the uses of CT?

Answer 14

Tumours, strokes, anatomical defects

Question 15

What are the advantages of CT?

Answer 15

• Tomographic (sliced bread)
• Can resolve grey and white matter, blood, CSF
• High resolution (0.5mm)

Question 16

What are the disadvantages of CT?

Answer 16

• Use of ionising radiation
• Some risk associated with injection of contrast (cancer)
• Anatomical images only (no function)

Question 17

Describe filtered back projection;

Answer 17

Technique used to generate the accurate 2D image and later 3D.

• Filter increases the resolution of the image

Question 18

Whats PET?

Answer 18

The use of gamma rays produced by radioactive decay to detect function

Question 19

Describe the physical basis of PET;

Answer 19

Physical basis;

• Radioactive isotopes of certain elements (CNOF) can be substituted into biologically important compounds (i.e glucose)
• Decay of these atoms release positrons
• Positrons collide with electrons and undergo an antimatter reaction to release two gamma rays 180 degrees apart
• Detection of gamma rays indicates position of labelled molecules

Question 20

Describe the procedure of PET;

Answer 20

• Isotope labelled compounds are injected or inhaled.
• i.e 2DG is taken up by neurons and phosphorylated preventing further metabolism, building up in the cell.
• Gamma rays can be detected by an array of crystal photomultipliers
• Images of activity are reconstructed using tomographic techniques

Question 21

What are the uses of PET?

Answer 21

• Glucose imaging of metabolism
• Blood flow imaging using labelled water (blood flow is good indicator of function)
• Image subtraction can highlight are associated with specific tasks

Question 22

What is image subtraction?

Answer 22

The process whereby you take an image of a brain at rest and subtract this activity from the active image to highlight the areas active for a specific task done in the active image.

Question 23

What are the advantages of PET?

Answer 23

• Brain function can be imaged

- Also distribution of receptors (e.g dopamine), blood flow, using labelled water

Question 24

What are the disadvantages of PET?

Answer 24

• Anatomical images not obtained

- Poor resolution 4-8mm (because positron can move some distance before striking electron)

Question 25

Whats a composite image?

Answer 25

An image generated from two imaging techniques i.e CT and PET

Question 26

What is effective dose?

Answer 26

The description of the radiation received per an imaging tehcnique.

i.e

• Tissue dependant
• X ray = 0.05 mSv
• Ct = 5-20 mSv
• PET/CT = 25 mSv

Question 27

What is an mSv;

Answer 27

Millisieverts, describes the probability of getting cancer from radiation exposure

1Sv = 5% chance