Advanced Imaging

Question 1

CT scan generations

Answer 1

1: Parallel pencil beam, single detector (slow, need to translate)
2: Fan beam, multiple detectors (faster than 1st, need to translate)
3: Wider fan beam, multiple curved detectors (faster, rotates, helical)
4: Same beam as 3rd, 360 fixed detectors, only source rotates (fastest, most expensive)

Question 2

CT pitch

Answer 2

Rate of movement
Table travel per x ray tube rotation/image thickness

Pitch = x means for xcm travelled by table the slice thickness is 1cm
So x»1 means poorer quality
x<1 means overlapping of anatomy + higher dosage, reduces diagnostic quality

Lower pitch = better spatial resolution

Question 3

CT vs CBCT

Answer 3

Components are similar

CT fan shaped but CBCT is coned shaped

CT rotates multiple times, CBCT only once

CT can use Hounsfield Units, CBCT cannot

CT scans have different windows (bone and soft tissue)

Question 4

CT collimation

Answer 4

Pre-patient: restricts X ray beam entering patient to minimize dose, altering thickness of cross-sectional slice

Post-patient: restricts X ray beams entering detector to reduce scatter and increase contrast

Question 5

Hounsfield Units

Answer 5

Every voxel on CT scans has a Hounsfield Unit number

HU are direct representations of density relative to water (HU 0) to characterize tissue and objects

High = white (bone, metal)
Low = dark (soft tissue)
Negative = darker than water (fat, air)

Question 6

CT windows

Answer 6

Bone window: trabecular bone clear, soft tissue unclear

Soft tissue window: trabecular bone unclear, soft tissue clear

Question 7

CT voxels

Answer 7

Non-isotropic unless thickness is ~1mm and pitch is less or equal to 1

Dependent on pitch as z dimension changes with image thickness

Question 8

FOV in CT

Answer 8

Doesn’t exist, instead described via ROI (e.g. face CT)

Question 9

CT display views

Answer 9

Orthogonal: axial, sagittal, coronal

MPR: oblique, surface volume render, MIP

Question 10

Pros of CT scans

Answer 10

MPR allows for viewing of all 3 planes w/o superimposition

Very good contrast resolution for visualization of bone and soft tissue details

Contrast agents can be used in contrast scan to visualize soft tissue and vessels

Question 11

Cons of CT scans

Answer 11

High radiation doses, avoid in pregnant patients unless emergency

Expensive

Contrast agents contraindicated in patients with iodine allergies and renal impairment

Question 12

Indications for face/head CTs

Answer 12

Less for dental, more for soft tissue/bone infection

1) Evaluation of pathology
2) Infections
3) Trauma evaluation
4) Craniofacial evaluation
5) Paranasal sinuses, temporal bone evaluation
6) Investigating headaches, strokes, brain injuries
7) Dental implant planning (But not rly used anymore)

Question 13

How does MRI work

Answer 13

Uses magnets so no radiation

Magnets align H nuclei to either magnetic north or south

Intermittent RF pulses are emitted by scanner at timings that can be changed

H nuclei respond by flipping, absorbing then releasing energy after flipping back

MRI coil detects those signals, processed by algorithms to produce an image

Different energy levels correspond to different greyness values on the image

Question 14

Different MRI protocols

Answer 14

T1-weighted +/- contrast:
> Black = air, bone
> White = fat, blood

T2-weighted:
> Black = air, bone
> White = fluid

Bright on T2 but dark on T1 = fluid, indicative of a cyst when its pathological

Question 15

Pros of MRI

Answer 15

No radiation
Excellent soft tissue detail
Using paramagnetic contrast agents can enhance soft tissues

Question 16

Cons of MRI

Answer 16

Expensive
Long scan times (15min-2h)
Susceptible to metal artefacts

Question 17

Indications of MRI

Answer 17

1) Soft tissue pathology
2) Malignant spread to lymph nodes
3) Perineural involvement
4) Inflammatory conditions like osteomyelitis
5) Vascular lesion evaluation
6) TMJ articular disc evaluation

Question 18

Contraindications of MRI

Answer 18

Metal implants
Metallic foreign bodies
Dental implants/braces not rly contra but can cause distortion
Pregnancy
Impaired renal function at risk of developing nephrogenic systemic fibrosis (due to MRI contrast agents)
Claustrophobia

Question 19

How does nuclear medicine work

Answer 19

Look at function instead of morphology
Can be combined with MRI or CT
Give px radionuclides which move into organs proportionally with tissue activity
Radioactive signals given off are measured, high signals = high activity
High glucose metab = high activity, e.g. tumours

Question 20

Types of nuclear medicine scans

Answer 20

Bone scans

Positron Emission Tomography (PET) scan

Question 21

What are bone scans used for?

Answer 21

> Syndromes w increased bone activity like Paget’s or condylar hyperplasia
Skeletal tumours/metastases
Infections like osteomyelitis, proliferative bone growth as a response to infection
Fractures

Dark black = higher activity

Spine darker is normal for older patients

Dispersed abnormal dark spots could be metastases

One spot lights up could be hypertrophy/hyperplasia

Question 22

PET scan

Answer 22

Uses FDG (a glucose analogue) tagged to radioactive molecule, taken up esp by cells metabolizing glucose at high rates

Combined with CT/MRI scans to provide anatomic localization w functional imaging

Allows for imaging of areas with high turnover or activity, esp for diagnosing/staging cancers and metastases and in infection

Question 23

What are extraoral skull projections?

Answer 23

Uncommon now bc can do CT scans
But can be used in emergencies

Basically a single beam of X ray at various angles wrt to skull

Question 24

Cons of extraoral skull projections

Answer 24

Sensitive to patient and beam positioning

Significant superimposition on the final image

Question 25

Types of extraoral skull projections

Answer 25

Lateral skull (like latceph, for skeletal/dental r/s, cranium, trauma, Paget’s disease)

PA skull/ceph/mandible (for skeletal asymmetry and trauma)

Oblique lateral mandible (for mandibular trauma and fractures)

Waters/occipitomental projection (for evaluating paranasal sinuses for sinusitis and zygomatic arch fractures)

Submentovertex projection (for evaluating anterior mand, condyle, skull base, zygomatic arch and trauma)

Reverse Townes projection (for evaluating condylar head/neck and trauma)