Extraoral Radiography 2 Flashcards
What is cephalometric radiography?
Lateral radiographic view of the skull.
Taken with x-rays aimed perpendicular to the midsagittal plane of the skull.
How must the midsagittal place be organized for cephalometric radiography?
It must be perpendicular to the floor.
What is cephalometric radiography used for?
Usually for orthodontic and/or orthognatic surgery cases.
It is reproducible because of a cephalostat.
What type of scan is cephalometric radiography comparable to?
This is the dental counterpart of the lateral skull radiograph in general radiology.
How is cephalostat done?
2 ear rods + 1 nose bridge
Central X-ray through the external auditory meatus or patient scanned from posterior to anterior.
Midsaggital plane should be perpendicular to the floor
Teeth in maximal occlusion
Neck straightened
What must be visible in a cephalometric radiograph?
Bones of the skull (sella turcica is used as a reference)
Soft tissues of the face
How is the image different between analogue and direct digital imaging of a cephalometric radiograph?
In analogue imaging, an aluminum wedge is placed in the area of the soft tissue profile which absorbs more X-rays, rendering the soft tissues visible on the radiograph.
In direct digital imaging, the aluminum wedge has been replaced by software enabling one to distinguish those soft tissues by automatic lowering of the kVp during exposure.
How can the aligment be tested in cephalostat imaging?
One ear rod will have a small metal ball and the other one a metal ring. When alignment is good then the ball should be in the middle of the ring.
What are the indications for using cephalostat?
Diagnosis of underlying skeletal pathology / aberrations
Diagnosis of jaw fractures
Diagnosis of underlying pathology or aberrations of soft tissues
Planning for orthodontic treatment / orthognatic surgery
Follow-up of any of the above indications.
Position assessment in the skull (often replaced by cone beam CT or MSCT)
How should head be arranged in anterior-posterior radiographs?
Nose-forehead towards the image receptor. Maxillary sinuses are more visible if the petrous part of the temporal bone being projected higher in the image
Chin-nose towards the image receptor if the petrous part of the temporal bone is being projected lower in the image to make orbits better visible.
Caveats for arrangement of anterior-posterior cephalostat radiographs:
L and R changes so check ear pods for correct orientation
How should head be positioned? (chin up or down)
Midsagittal plane of skull should be perpendicular to the floor
What are the important reference points that are important for cephalometry?
Sella turcica (S)
Orbitale (infra-orbital rim)
Nasion (most anterior point of the naso-frontal suture)
ANS (Anterior nasal spine)
Prosthion
Point A or subspinal point (A)
Infradental point (Id)
Pogonion (Pog)
Supramental point or point B (B)
Menton (Me)
Gnathion (Gn)
Gonion (Go)
PNS
Articulare (Ar)
Porion (Po)
Where is the infradental point located?
Most anterior point of the alveolar crest of the mandible near the incisors (Between the 2 central incisors)
Where is the pogonion located?
Most anterior point on the cortex of the chin
Where is the supramental point located?
Deepest point in the anterior cortex of the mandible. (Deepest point between Id and Pog)
Where is the menton located?
Lowest point of the mandibular cortex near the symphysis
What is the gnathion?
Most anterior and inferior point of the cortex of the mandible near the chin (middle between Pog en Me)
What is the gonion?
Most lateral external point of the angle of the mandible.
(Draw a bisecting line between the line tangient to the lower border of the mandible and the line tangent to the posterior rim of the ramus of the mandible.)
Where is the PNS?
Most posterior point of the floor of the nose / hard palate
What is the articulare?
Crossing between the tangient line of the ramus of the mandible and the line tangient to the lower border of the sphenoid bone.
What is the porion?
Most superior point of the external acoustic meatus - should/could coincide with the ear rod shadow of the cephalostat
What 2 reference points does the frankfort line pass through?
Po and Or
What reference points does the maxillary plane pass through?
ANS and PNS
What reference point does the mandibular plane pass through?
Tangient line to the inferior border of the mandible. Goes through Gn and Go
Also through Me and Go
What reference points does the SN plane run through?
S and N. It is used to calculate the angle between SN and point A (relative position of the maxilla in relation to the anterior base of the skull)
What is the angle between SN and point B?
The relative position of the mandible in relation to the anterior base of the skull (SN plane)
What is the angle ANB?
Assessment of the position of the maxilla in relation to the mandible - skeletal class 1, 2, and 3 profile classification.
What is the inclination of the maxillary incisors on the cephalometric image?
Angle between long axis of the central maxillary incisors and the SN plane.
What is the inclination of the mandibular incisors on the cephalometric image?
Angle between long axis of the central mandibular incisors and the mandibular plane.
What is all on one radiograph used for?
It is done for orthodontic planning or orthognatic surgery planning.
Used to be manual but these days software programs are available that help calculating angles and drawing appropriate lines through different reference points.
What are the features of cone beam computed tomography?
Cone shaped (pyramid) radiation beam
Intermittent beam
Single rotation / scanning trajectory (360 - 270 - 180)
Cylinder or sphere shaped volume
Isotropic voxels
Adjustable mA and kV and time
Adjustable field of view
No differentiation between soft tissues possible
No Hounsfield units (only in medical CT)
Patient sitting or standing
How should field of view be arranged in CBCT?
Should be as close as possible to the region of interest
Position of the rotation center in the correct place is crucial.
What does scout view do?
Offers check of correct positioning
What does spatial resolution depend on in CBCT?
Voxel size
What determines the nominal voxel size?
Matrix of the image detector and the size of its pixels. (Smaller pixels capture fewer X-rays and result in higher noise in the image)
What determines geometric un-sharpness on CBCT?
The focal spot size of x-ray source
Geometric configuration of x-ray source
Object–to detector distance
Patient movement
Type of scintillator in the detector
Image reconstruction algorithms
What does contrast represent on CBCT?
Ability to show differences in attenuation
How are patients selected for CBCT?
Must be able to cope with procedure and remain still during the entire exposure
2D images were inconclusive and 3D will add crucial diagnostic information
No soft tissue pathology
Bone and teeth involved
Dento-alveolar trauma
Jaw fracture
Complicated endodontic case
Complicated orthodontic case
Implant placement planning
Jaw bone pathology
Supernumerary teeth and complicated eruption issues
38/48 extractions (molar-nerve canal relationships)
TMJ issues (not including the discs)
Follow-up surgical treatment in the jaws
What are the contraindications for CBCT?
Patient cannot cope with the procedure
Soft tissue pathology
TMJ disc visualization
Standard endodontic or orthodontic planning
Screening new patients
What factors create artefacts in CBCT?
Beam projection geometry
Trajectory of the rotational arc
Image reconstruction algorithms
The above cause scatter, partial volume averaging, and the cone beam effect.
What is scatter? What effect does it have on the image?
Diffracted X-ray photons due to interaction with matter.
Scattered photons will be captured by image detector and will continue fogging of the image (quantum noise)
This degrades the image quality and causes streaking artefacts which is similar to beam hardening effects.
What is partial volume averaging?
This phenomenon occurs in CBCT and occurs when voxel size is larger than the size of the object being imaged. This leads to pixel showig the average grey value of the 2 points within the voxel due to the voxel representing one pixel of contrast
How is the partial averaging phenomenon solved?
Selecting the smallest voxel size if possible.
What is the cone beam effect?
This occurs in the peripheral areas of the scan, due to the divergent X-ray beam which rotates around the patient’s head in the horizontal plane. Therefore the structures at the top of the image are only exposed when the X-ray source is opposite side of the patient
What is the product of the cone beam effect?
It results in streaking, distortion and peripheral noise
What are the types of inherent CBCT artefacts related to the procedure?
Under-sampling: Too few basis projections captured or an incomplete arc trajectory.
Ring or circular streaks: From imperfections in scanner detection or poor calibration
Double contour artefact: Misalignment of the X-ray source and the detector (shows up like patient movement)
What are the introduced artefacts that can arise from CBCT?
Beam hardening: When X-ray photons pass through dense matter and the weaker photons are absorbed.
Cupping artefact caused by metallic structures absorbing more X-rays)
Streaks and dark bands seen between 2 dense objects resulting in missing or extinction of value artefacts.
How can introduced artefacts be reduced?
Using a smaller FOV
Remove all metallic objects/jewelry in the head and neck area even if they are not in the FOV
What type of artefacts are caused by the patients themselves?
Patient motion such as heavy breathing, heavy heart beat, swallowing, and accidental movement.
This results in double contours.
What causes the patient related artefacts to be exacerbated?
The smaller the voxel size the larger the effect of movement.
What is the effect of a larger FOV on the patient?
The larger the FOV, the higher the radiation dose for the patient
FOV should be as close to the region of interest as possible.
Usually a smaller FOV is combined with a high resolution.
How is FOV different between large, medium, and small images?
Large uses stitched images to form a complete skull
Medium uses the images to form an image of just the mouth
Small shows teeth alone often.
How is a medical CT different to the cone beam CT?
MSCT: Spiral motion of a fan shaped beam with continuous radiation. Stack of slices formed, collected, and provides 3 dimensional image.
CBCT: Cone shaped beam, takes intermittent images around the patient.
What effect does modifying voltage have on imaging risk?
Too low of a kV can cause significant increase in absorbed dose for the patient, too high of a kV may over radiate the patient, the optimal kV i usually determined by the manufacturer.
What effect does current have on the imaging risk?
Amperes are directly proportionate to the radiation dose.
Changing the mA has direct implications fro image quality as noise will increase when the mA is decreased.
What effect does exposure time have on the imaging risk?
Exposure time is also directly proportionate to the radiation dose.
Decreasing time also deteriorates image quality slightly.
Decreasing the time may benefit patients who cannot sit still for long. This will ofc have consequences on image quality and diagnostic yield.
What kind of voxels are used in modetn CTs?
Isotropic voxels are used instead of anisotropic voxels (older multi slice CTs use anisotropic voxels)
What causes artefacts on CBCT?
Metal artefacts (Crowns, amalgam, posts, foreign objects)
Gutta percha (Not all types give the smae artefacts and not all sealers give the same artifacts.)
Movement or motion (heavy breathing, heavy heart beat)
What benefits can arise from thyroid shielding?
It can reduce the thyroid dose by 50%
What should be done to CBCT image after it is taken? (steps before interpretation)
- Check for image quality
- Re-orient the data if necessary (e.g. ANS –PNS
horizontal) - Optimize the data (contrast, brightness, …)
- View the data systematically in all 3 planes (axial, coronal and sagittal) = cine mode /
paging mode - Format the data (multiplanar
reformation/reconstruction or MPR, ray sum and volumetric rendering) - Interpretation report
What are the benefits of using multiplanar reformation of a CBCT?
Allows the jaw to be viewed panoramically allowing transverse or cross-sectional sections to be viewed in a separate window.
Ideal for visualizing the teeth and jaw bone
Alignment of structures under review with section plane is important for correct diagnosis
What is ray sum view of CBCT?
Thickening the slice thickness of the MPR images
Ideal for cephalometric reconstruction of the scan volume.
What is the problem with ray sum reconstruction?
Increases anatomical noise in the image due to superimposition of anatomical structures.
What is volumetric rendering used for?
Ideal for visualization of teeth in an orthodontic case.
Filters determine the result so careful when interpretting bone lesions on these images.
Can be solid volume rendered image or a transparent volume rendered image.
What is a dicom file?
Digital imaging and communications in medicine.
What is dicom used for?
Can be exchanged with other care providers if they have a DICOM-viewer
Can be exchanged with dental labs to create surgical guides or 3D models.
LOOK AT THE FOLLOWING TABLES IN THE EXTRAORAL RADIOGRAPHY 2 SLIDES:
CBCT VS MSCT
CBCT VS OPG
