Advanced Imaging

Question 1

Describe how a CT image is acquired

Answer 1

• xray beam and detector continuously revolve as the table moves patients through the gantry
• rate of movement can also be described as pitch (lower pitch -> better spatial resolution)
• BUT if pitch <1 = overlapping anatomy and higher dose to patient
• CT beam (fan shaped)

Question 2

What is the function of pre-patient and post-patient collimator

Answer 2

Pre-patient:
- restricts x ray beam that enters patient’s body
- specifies thickness of cross-sectional slice
- minimises radiation dose to patient

Post-patient:
- restricts x-ray beam from entering detector assembly at arbitrary angles
- reduces the amount of scatter radiation, improving image contrast

Question 3

Describe the principles of CT image acquisition

Answer 3

• x ray beam exits patient -> captured by detector on opposite side
• computer algorithms known as filtered back-projection algorithms are used to reconstruct the image
• each pixel has a ‘CT number’ measured in Hounsfield unit (helps to characterise tissue and objects)

Question 4

Strengths and Limitations of CT scans

Answer 4

Strengths:
- CT scans’ MPR (multiplanar reformation) allows viewing in all 3 planes without superimposition
- very good contrast resolution (can see bone and soft tissue in detail)
- use of contrast agents can enhance soft tissues and vessels

Limitations:
- high radiation doses to patients (avoid in pregnant patients unless emergencies)
- expensive
- contrast agents are contraindicated in patients with iodine allergies and renal impairment

Question 5

Indications for CT Face

Answer 5

1) evaluation of pathology (cysts, benign and malignant tumours)
2) infections (both soft tissue and osteomyelitis)
3) evaluation of trauma
4) craniofacial evaluation
5) evaluation of paranasal sinuses and temporal bone
6) investigating symptoms of headaches, stroke, brain injuries

Question 6

Principles of MRI imaging

Answer 6

• uses magnets not ionising radiation
• magnets in the MRI scanner align the hydrogen nuclei in the body to its magnetic field
• Radiofrequency pulses are emitted by the MRI scanner intermittently
• Hydrogen nuclei respond to the RF pulses by absorbing and releasing energy respectively
• MRI coil detects these signals, which are processed by algorithms to form the MRI image

Question 7

Difference between T1 and T2 weighted sequence

Answer 7

T1:
- BLACK: air, calcium, cortical bone, rapidly flowing blood
- high SI: fat, blood = contrast

T2:
Black: same
High SI: fluid, CSF, bladder, bile/gallbladder, kidneys

Question 8

Strengths and Limitations of MRI

Answer 8

Strengths:
- no ionising radiation
- excellent soft tissue detail
- use of paramagnetic contrast agents can enhance soft tissues

Limitations:
- expensive
- long scan time (15 mins to 2h)
- also susceptible to metal artifacts

Question 9

MRI Contraindications

Answer 9

• patients with metal-containing implants (pacemakers, defibrillators, cochlear implants, joint replacements)
• metallic foreign body (bullets, body piercings)
• dental implants and braces can cause image distortion (may need to remove braces)
• always check with physician/radiology department
• pregnancy
• patients with impaired renal function at risk of developing nephrogenic systemic fibrosis
• claustrophobia

Question 10

Indications for MRI in H&N

Answer 10

1) Soft tissue pathology (tongue, cheek, salivary gland, neck)
2) Malignant spread to lymph nodes
3) Perineural involvement
4) Edematous changes in inflammatory conditions (osteomyelitis)
5) Evaluating vascular lesions
6) Evaluation of TMJ articulated disc

Question 11

Describe nuclear medicine

Answer 11

• includes bone scans and PET scans
• looks more at function than at anatomy
• uses radionuclides (gamma-emitting radioactive molecules) which are taken up into various organs
• radioactivity of radionuclides are measured by detectors (high signal = high activity)

Question 12

Describe PET scans

Answer 12

• position emission tomography
• uses FDG (a glucose analogue) -> taken up by glucose-metabolising cells
• PET scans can be fused with CT or MRI scans to provide anatomical localisation
• used for diagnosis and staging cancers and metastases
• infection/osteomyelitis

Question 13

List the types of extra oral skull projections

Answer 13

1) Lateral
- skeletal and dental relationships, evaluate cranium, trauma
2) PA skill/ceph/mandible
- skeletal asymmetry
- trauma
3) oblique lateral mandible
- trauma
4) waters
- paranasal sinuses and zygomatic arch
- trauma
5) Townes projection
- condylar head and neck
- trauma
6) submentovertex projection
- anterior mandible, condyle, skull base, zygomatic arch
- trauma