Radiology Flashcards
What is the basic principle of CBCT ionising radiation?
conical/pyramidal X-ray beam and square digital detector rotate around head (no more than one full rotation), captures many 2D images which are reconstructed into a 3D image
How is a patient usually positioned in a CBCT machine?
usually either standing or sitting
rarely supine
What are the horizontal and vertical planes of CBCT head positioning based upon?
horizontal - frankfort plane (ie head level with ground)
vertical - midsaggital plane
What are the advantages of CBCT over plain radiography?
- no superimposition
- ability to view subject from any angle
- no magnification/distortion
- allows 3D reconstruction
What are the disadvantages of CBCT compared to plain radiography?
- increased radiation dose
- lower spatial resolution
- susceptible to artefacts
- expensive equipment
- imaged more complicated to interpret
- requires additional training
What are the benefits of a CBCT compared to conventional CT scan?
- lower radiation dose
- potential for sharper images
- cheaper
- smaller footprint
What are the main benefits of conventional CT compared to CBCT?
- able to differentiate soft tissues better
- “cleaner” images
- large field of view possible
What are the common uses of CBCT in dentistry?
- relationship btwn M3M and IANC prior to intervention
- alveolar bone dimensions to plan implant placement
- visualising complex root morphology for endo
- investigating external root resorption next to impacted teeth
- assessing large cystic jaw lesions and involvement of anatomical structures
What are the three common orthogonal planes?
- axial (above or below)
- sagittal (split down middle into L and R)
- coronal (front or back, shoulder to shoulder)
What are the uses of 3D volume reconstruction?
- help clinician picture extent/shape of disease
- can be informative teaching aid for patient
What are the drawbacks of 3D volume reconstruction?
it is a modified reconstruction of the data and so can create misleading images - particularly poor at showing thin bone
What is the field of view?
size of the captured volume of data
decision based on clinical case
What does an increased FOV also increase?
increases radiation dose, number of tissues irradiated an increased scatter
What is a voxel?
3D pixel
What does voxel size impact?
image resolution
What does a decreased voxel size do?
leads to increased radiation dose and scan time
What imaging factor choices would you make for an endodontic case?
FOV as small as possible unless large apical pathology
small voxel size to see canals in detail
What imaging factor choices would you make for an implant treatment planning case?
FOV depends on number/position of implants
larger voxel size (fine detail not as necessary)
What factors can impact radiation dose?
equipment, FOV, position of FOV, voxel size
Dento-alveolar CBCT is roughly what multiplication of the radiation dose of a panoramic radiograph?
2-3x dose of panoramic radiograph
What are artefacts?
visualised structures that were not present in object investigated
most can be avoided
What are the two main types of artefacts?
1) movement artefacts
2) streak artefacts
What is a movement artefact and how does it occur?
occurs if patient not completely still during full exposure
leads to general blurriness or extra contours
How are movement artefacts avoided?
reduced using fixation aids - chin rest, head strap etc
What are streak artefacts caused by and what are their main issues?
- most notably caused by high-attenuation objects, primarily metals e.g. amalgam
- can prevent caries assessment adjacent to restorations
- can prevent assessment of perforations/missed canals in root treated teeth
What are the contraindications for CBCT?
- if plain radiographs are sufficient
- pathology requiring soft tissue visualisation eg. malignancy, infection spreading in soft tissue
- high risk of debilitating artefacts
- patient factors - unable to sit still, unable to fit in machine (kyphotic pt or unfavourable neck to shoulder ratio)
Are UK undergraduates qualified to carry out CBCT?
Current UK teaching deemed inadequate to refer, justify, perform or interpret CBCT
Name 5 other imaging modalities other than CBCT
1) CT conventional
2) MRI
3) Ultrasound
4) nuclear medicine
5) combination of multiple imaging modalities e.g. PET-CT, PET-MRI
What is a CT (conventional) scan and what can be done to create contrast in the image produced?
- cross-sectional imaging which allows soft tissue and bony anatomy to be visualised well.
- may need iodinated contrast intra-venously to increase soft tissue contrast
What are CT images constructed using?
voxels
What kind of beam is used in CBCT?
Cone shaped beam
What kind of beam is used in CT?
fan shaped beam
What has a higher radiation dose, CT or CBCT?
CT
What is the soft tissue contrast like in CBCT?
poor soft tissue contrast
What is the soft tissue contrast like in CT?
good soft tissue contrast
Is a IV radiographic contrast used in CBCT?
no
How is a patient positioned for CBCT?
upright or sitting
How is a patient positioned for CT?
lying horizontal
CT voxels are given a value of density termed what?
Hounsfield units
Newer CT units utilise what kind of voxels?
isotropic voxels - allow manipulation of images in all three planes (axial, coronal and sagittal) with no distortion
What is air’s hounsfield number?
-1000, appears black
What is fat’s hounsfield number?
-100 to -60
What is the hounsfield number of water?
0
What is the hounsfield number of dense bone?
+1000
What is windowing?
alters levels of densities visualised in scan to optimally look at specific tissues
window level = value at centre of range
window width = range of values selected to view tissues
What is the magnetic field for an MRI?
the entire room
imaging coil placed over region of interest to gain higher resolution
In an MRI, what colour is cortical bone?
black
In an MRI, what colour is fat and fluid?
white
What are the advantages of an MRI?
no ionising radiation
very good imaging of soft tissues and early changes in bone marrow
What are the disadvantages of an MRI?
- multiple contraindications
- lengthy scans
- claustrophobia
- cost and availability
What are the contraindications for MRI?
- pacemakers
- orbital foreign bodies
- artificial heart valves
- surgical clips
Why are implanted metals in the body a danger for MRI scanning?
metal attracted to magnetic field which means any metal in patient can be caused to move through the tissues causing pain or potentially fatality
What are three examples of the uses of MRI in dentistry?
1) MRI TMJs - visualise cortical bone and articular disc in joint space (displacement, degenerative changes etc)
2) MRI salivary glands - detect masses
3) MRI sinus for malignancy - masses in sinuses
How is an ultrasound scan carried out?
- high frequency sound waves used to produce images
- good for superficial structures
- transducer placed on skin surface with coupling agent (gel) between probe and skin to allow soundwaves to penetrate
How does an ultrasound scan produce an image and what can they not penetrate?
as sound waves hit tissue boundaries, they reflect back to probe
length of time to return creates level of depth
sound waves cannot penetrate bone
What are the advantages of an ultrasound scan?
- no ionising radiation
- good for superficial structures
- real time imaging
- can be used as adjunct to biopsy/aspirates
- blood flow
What are the disadvantages of an ultrasound scan?
- operator dependent
- can’t penetrate bone
- can be difficult to interpret
- only superficial structures seen
When can ultrasound scanning be utilised in head and neck?
- neck lumps - cysts, tumours, lymph nodes
- salivary glands - Sjogrens, neoplasia, sialadenitis, sialoliths
- guidance for biopsy
- vascular - carotid artery stenosis, vascular lesions
What are three examples of nuclear medicine?
1) PET (positron emission tomography)
2) SPECT (single photon emission CT)
3) scintigraphy
How does nuclear medicine generally work?
patient typically injected intra-venously with radiopharmaceutical which when it decays (via half life) emits particles that are picked up on gamma camera
- increased activity (hot spot)
- reduced/no activity (cold spot)
What are the advantages of nuclear medicine scans?
- can be superimposed over other cross sectional imaging (conventional CT)
- highly sensitive
- can assess function
What are the disadvantages of nuclear medicine scans?
- poor resolution
- poor specificity
- very high radiation dose
What radioisotopes can be used in nuclear medicine?
- Technetium - short half life (6hrs), can be bound to different substances that are taken up by specific tissues, MDP (bone) and pertechnetate (thyroid and salivary glands)
- 18-FDG - fluoro-deoxyglucose (fluorine labelled glucose), short half life (just under 2hrs), gets taken up by metabolically active tissues
What is a PET-CT scan?
positron emission tomography which is a type of nuclear medicine, superimposed over a conventional CT.
PET gives high sensitivity for increased glucose uptake
CT gives high resolution for anatomical detail
What is a PET-CT scan typically used for?
- cancer diagnosis
- unknown primary disease ie. pt with solitary neck lump and nothing on clinical exam
- can be used to assess recurrence, to assess effectiveness of treatment and to distinguish between active vs scar tissue
