Alternative imaging modalities

Question 1

Give examples of other modalities?

Answer 1

• MRI (magnetic resonance imaging)
• ultrasound
• nuclear medicine (radio-isotope scanning)
• combination of modalities including PET

Question 2

Voxels in CT, CBCT & MRI

Answer 2

· Patient divided into voxels (pixels with volume)

· Each voxel is given CT (Hounsfield) number according to amount it has attenuated beam.

· Different CT numbers given different shade of grey.

· Bone white, soft tissue grey and air black (like X‐ray)

· Can manipulate image to allow better visualisation of more subtle changes between tissues (windowing)

Question 3

CT blow out fracture - changing the windowing

Answer 3

Blow out fracture.

· The left image has soft tissue windowing, and the muscles of the eye can be seen clearly.

· When a patient has a fracture of the orbital floor – blow out fracture – the orbital contents can herniate into the sinus. The muscles can be trapped, and the movement of the eye be restricted.

· The right image has hard tissue windowing and shows the bone more clearly.

Question 4

advantages of CT

Answer 4

· Images bone and soft tissue

· Differentiate different tissues.

· Head injury – can see bone and brain.

· Speed (compared MRI)

· Multi-planar (manipulate, reconstruct images)

· Cost and availability usually cheaper (compared with MRI)

Question 5

disadvantages of CT

Answer 5

· Patient Radiation Dose

· Dose for head ~2 mSv

· Often requires intravenous contrast to distinguish tissues (iodine based, allergy)

· Artefacts (metallic objects, amalgam in head and neck)

· Expensive in comparison with plain radiography

Question 6

describe MRI

Answer 6

· Theory difficult to understand.

· Involves protons (water), magnetic field, radiofrequency pulses.

· Patient is placed in a magnetic field.

· The water protons align in this field.

· Another magnetic field is applied at an angle & then removed.

· The protons oscillate/resonate back to their original position.

· This resonance is measured by the computer.

· Good for looking at soft tissue.

· Cortical bone and dental hard tissue always black – not much water present

· Various sequences show different tissue as bright/dark.

· T1 weighted – water dark.

· T2 weighted – water bright.

· Generally, T1 for anatomy, T2 for pathology

· Other sequences can be carried out.

Question 7

advantages of MRI

Answer 7

· No ionising radiation

· Excellent for viewing soft tissue.

• cancellous bone
• changes in marrow, infection, infiltration
• cortex breach

· Multi-planar

Question 8

disadvantages of MRI

Answer 8

· Multiple contraindications

• Pacemakers
• artificial heart valves
• surgical clips particularly intra-cerebral aneurysm clips
• 1st trimester pregnancy

· FB within the eyes/orbits

· Danger of strong magnetic field – might clear your bank card!

· Units need to be away from car parks & other facilities.

· Availability and cost of scanning – waiting lists.

· Scan times can be quite long and noisy.

· Claustrophobia

Question 9

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