Panoramic radiography Flashcards
describe panoramic radiography
- designed to provide a clear view of the entire maxillomandibular region
- proprietary (trade names) - Orthopantomogram (OPT/OPG), Panorex
What is the problem of superimposition?
Radiographs produced by passing X-ray beam through everything between X-ray source & receptor so all these structures will be overlaid on the 2D image, potentially obscuring each other. [sinuses, nose, tongue etc all in the same area]. This is less of an issue with intra-oral radiographs as X-ray source & receptor can both be placed close to area of interest (therefore fewer structures captured).
What is the solution - tomography?
Allow “slices” of the subject to be viewed separately
Describe tomography in medical imaging
1) Conventional - one slice (mostly outdated in medicine except for panoramic radiographs)
2) Computed - multiple slices (all others)
Can be produced using different phenomena?
- X-rays - e.g. panoramic radiographs, CBCT, CT
- Radio waves - e.g. MRI
- Gamma rays - e.g. SPECT (Nuclear Medicine)
- Positrons - e.g. PET (Nuclear Medicine)
Why panoramic radiography?
Form of conventional tomography which was developed to capture a curved slice aligned with the “horseshoe” shape of the jaws and then displayed as a 2D flat image
describe exposure technique
- the area of interest is exposed sequentially from one side to the other over an extended time (e.g. 14 seconds)
- this contrasts with intra-oral radiographs for which the area of interest is exposed uniformly in a split second (e.g. 0.2 seconds)
- this greatly alters how the image is affected by patient movement
describe conventional tomography technique
panoramic radiography involves a modified version of linear tomography.
linear tomography captures a single, flat slice by moving the X-ray source & receptor past the area of interest during the exposure (should understand linear to grasp panoramic)
Describe linear tomography - principle of slice formation
X- ray source moves in one direction while receptor moves in opposite direction.
Structures in a “focal” slice remain projected onto same point of receptor.
Structures outside this slice are continually projected onto different points of receptor
Structures in the “focal” slice will appear clearly and be distinguishable on the image (the tooth). This plane is in the ‘focal trough’.
Structures outside this slice will appear faint & spread out across the image (tongue and lips are stretched out and not that clear) The further out, the worse the effect
Focal trough in Panoramic radiography = the tomographic slice of interest = jaws (mandible and teeth)
Structures outside this slice appear faint, out of focus and should not obscure the region of interest.
Slice (focal trough) is curved because of complex rotational movements of the X-ray source & receptor around the patient as they are moving around the head.
The point of rotation is not constant and mimics shape of the “average” mandible.
Patients with “non-standard” arch size/shape may not conform → reduced image quality.
What are focal trough limitations ?
Ectopic teeth may be far enough out of focal trough so they appear “missing”.
What is the aim of the orthogonal program?
Aims to provide an optimal view of the dentition
x-ray beam angulation changed to be more orthogonal (i.e. closer to 90 degrees) to the teeth.
Orthogonal should be used as a default choice unless clinical reason to change it
What is the advantages of orthogonal program?
- reduces overlap of the teeth to aid assessment of approximal caries
- particularly improves view of premolars (where dental arch curvature can be pronounced)
- Improves angulation to more accurately represent interdental periodontal bone levels
What is the disadvantages of orthogonal program ?
- Distorts rest of skeleton to varying degrees (e.g. maxillary sinuses, mandibular rami)
- typically, a narrower field of view so may miss condyles at edge of image
suitable for cases requiring only caries &/or periodontal bone loss assessment
