Advanced Imaging Flashcards
CT scan generations
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)
CT pitch
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
CT vs CBCT
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)
CT collimation
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
Hounsfield Units
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)
CT windows
Bone window: trabecular bone clear, soft tissue unclear
Soft tissue window: trabecular bone unclear, soft tissue clear
CT voxels
Non-isotropic unless thickness is ~1mm and pitch is less or equal to 1
Dependent on pitch as z dimension changes with image thickness
FOV in CT
Doesn’t exist, instead described via ROI (e.g. face CT)
CT display views
Orthogonal: axial, sagittal, coronal
MPR: oblique, surface volume render, MIP
Pros of CT scans
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
Cons of CT scans
High radiation doses, avoid in pregnant patients unless emergency
Expensive
Contrast agents contraindicated in patients with iodine allergies and renal impairment
Indications for face/head CTs
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)
How does MRI work
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
Different MRI protocols
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
Pros of MRI
No radiation
Excellent soft tissue detail
Using paramagnetic contrast agents can enhance soft tissues
Cons of MRI
Expensive
Long scan times (15min-2h)
Susceptible to metal artefacts
Indications of MRI
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
Contraindications of MRI
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
How does nuclear medicine work
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
Types of nuclear medicine scans
Bone scans
Positron Emission Tomography (PET) scan
What are bone scans used for?
> 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
PET scan
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
What are extraoral skull projections?
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
Cons of extraoral skull projections
Sensitive to patient and beam positioning
Significant superimposition on the final image
Types of extraoral skull projections
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)
