Radiology 18 Flashcards
Examples of other imaging modalities
MRI (Magnetic Resonance Imaging)
Ultrasound
Nuclear Medicine (radio-isotope scanning)
Combination of modalities including PET.
How are images created from voxels?
- Patient divided into voxels (pixels with volume)
- Each voxel is given CT (Hounsfiled) 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)
Features of CBCT voxels
Volume element
Pixel with depth
Size varies
0.08mm – 0.4mm
Smaller voxels result in a higher resolution image
Small size requires longer scan time
This results in higher dose
Image view?
axial
Image view?
coronal
Image view?
sagittal?
Basics of MRI
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
Cortical bone and dental hard tissue always black – not much water present
Various sequences show different tissue as bright/dark
Advantages of MRI
- Good for looking at soft tissue
- No ionising radiation
- Excellent for viewing soft tissue also cancellous bone:- changes in marrow, infection, infiltration cortex breach
- Multi-planar
Disadvantages of MRI
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
What is being shown here?
MRI benign parotid tumour
What is being shown here
MRI of TMJ
When is MRI used at DDH?
TMJ disorders – particularly disc problems
Salivary gland pathology – including MRI sialography
Assessing early bone changes in patients suspected of having Medication-related osteonecrosis of the jaw (MRONJ)
Basics of ultrasound
High frequency sound waves >13MHz
Transducer placed on skin
Sound waves bounce off tissues and back to the probe
Cannot travel beyond hard tissues
Limited use around the jaws
Advantages of ultrasound
No ionising radiation
No known harmful effects
Ideal for superficial soft tissue structures
Multi-planar
Operator dependent
Real time images
Blood flow
Can be used to guide fine needle aspirate or biopsies
Disadvantages of ultrasound
Operator dependent
Difficult to interpret
Superficial tissues
Cannot penetrate bone
Radioisotope scanning
Inject Isotopes which are unstable
and decay emitting radioactive
particles (a, b) or radiation (gamma)
Isotope selected according to tissue
to be imaged
Radioactive compound concentrated in target tissue indicating:
Increased activity (hotspot)
Reduced/no activity (cold spot)
Radioactive emissions detected by
gamma camera
Technetium (99mTc)
is a radionuclide nuclear agent
Short half life (6 hours) ensures lower patient dose
Easily available – currently! (BREXIT)
Can be bound easily to different substances
MDP – methylene diphosphonate – bone
Red blood cells
Taken up by thyroid and salivary glands
roblems with nuclear medicine imaging
Poor resolution
Appearances not specific and may not be able to distinguish between different pathological processes
Radiation dose
PET CT
Positron emission tomography combined with CT
Images are acquired with both technologies at the same time and superimposed on each other
The radio-active contrast can be given orally, inhaled or into a vein, depending on what is being investigated
This shows up areas where cells are more active than normal
Scans do not have as much fine detail as CT alone
