Chapter 9: Other Imaging Modalities Flashcards
The other imaging modalities include:
6:
1. Conventional tomography
2. Computed tomographic scanning (CT scan)
3. Cone beam conventional tomography (CBCT)
4. Magnetic resonance imaging (MRI)
5. Ultrasonography
6. Sialography
Conventional tomography:
Radiographic technique designed to image a slice or plane of tissue
- it’s accomplished by blurring the images of structures lying outside the plane of interest through the process of motion
Focal plane tomography:
- in conventional tomography
- same as focal trough
- tomography by simultaneously moving the xray generator and xray detector so as to keep a consistent exposure of only the plane of interest during image acquisition
Conventional tomography: what are the two objects involved
X-ray tube and radiographic film
The fixed axis in the conventional tomography is
Fulcrum
Principle of conventional tomography:
Moves in opposite directions simultaneously
Where is the X-ray tube and film positioned in conventional tomography?
On opposite sides of the fulcrum (located within the body’s plane of interest)
The images of objects outside the focal plane are?
Blurred
Tomographic layer is the zone of?
Sharp
Blurring in conventional tomography depends on?
- structures lie far from FP
- FP lies far from the film
- distance of the tube
Indications of conventional tomography?
- frontal plane: maxillary sinus, ethmoid bone, conchae
- sagittal plane: TMJ
Computed tomographic scanning (CT):
X-ray tube that emits a finely collimated FAN shape xray beam directed through a patient to a series of scintillation detectors or ionisation chambers
What do the detectors in CT scan do?
Measure the number of photons that exit the patient
CT scan produces what kind of image?
Cross sectional
What are the two objects involved in CT scanning?
- X-ray tubes and detectors
X-ray tubes in CT scans:
- tubes with rotating anodes
- high heat capacity
- 120 kVp/ 200-800 mA
- beam collimated to a thin FAN beam
Detectors of CT scans:
- array of solid detectors
- ceramic or crystal
- photodiode
- the signal from the detector is amplified, digitised, and sent to a computer for analysis
Image reconstruction of CT scan:
- photons recorded by receptors
- cross sectional images constructed by computer algorithms
- CT image is recorded as individual VOXELS
- 512 x 512 pixels or 1024 x 1024 pixels
Advantages of CT scans:
- eliminates superimposition
- high contrast resolution
- multi planar reformatted image: from a single CT can see axial, coronal , and sagital planes
- 3D images
Disadvantages of CT scans:
- expensive, maintenance
- metallic restorations
- patient motionless during examination
Applications of CT scans:
- infections
- osteomyelitis
- cysts
- tumours
- maxillofacial trauma
- implantology
-maxillary sinuses - salivary glands
- TMJ
- cleft palate
Cone beam computed tomography (CBCT):
- performed using a rotating platform
- divergent cone shape or pyramidal source of radiation is directed through the region of interest and the residual attentuated radiation beam is projected onto a detector on the opposite side
- the xray source and detector rotate around a rotation center
Difference between cone (CBCT) and fan shaped (CT)
- cone: reduces range, narrow xray beam by reducing ET, more accurate images
ET is up to ___% less than scanning time
50
The dimensions of the filed of view depend on the ?
Detector size and shape, projection geometry and collimation
CBCT takes ____ seconds and ____ individual projections frames
20, 600
Patient selection criteria for CBCT
Radiation doses higher than other dental Radiographic procedures
Specific applications of CBCTs:
- implant site assessments
- endodontics, orthodontics + 3D cephalometry
- impacted teeth
- TMJ
- maxilofacial pathoses
- treatment planning and virtual simulations
Strengths of CBCTs:
- size and cost: available for dental office
- fast acquisition: 30 seconds
- submilimeter resolution
- relatively low radiation doses
- interactive analysis
Limitations of CBCTs:
Image noise
Poor soft tissue contrast
Magnetic resonance imaging (MRI)
Patient is placed inside a large magnet, this magnetic field causes the nuclei of many atoms in the body to align with the magnetic field
Scanner directs a radio frequency pulse into the patient, causing hydrogen nuclei to absorb energy, the stored energy is then released and detected as a signal in the coil in the scanner, the signal is then used to construct an image
Is MRI an ionising technique?
No
Which of the imaging modalities is used in TMJ in dentistry ?
MRI
Advantages of MRI?
Soft tissue best contrast resolution
No ionising radiation is involved
Multi planar imaging without reorienting the patient
Disadvantages of MRI
Long imaging times
Expensive
Patients with ferromagnetic metals (cardiac pacemakers, cerebral aneurysm clips)
Applications of MRI:
- soft tissue conditions
- disk in TMJ
- tumours
- lymph nodes
- mandibular nerve
Ultrasonography:
- no radiation
- based on sound waves that acquire images in real time without ionising radiation
- scanners generate electrical impulses that are then converted into ultra high frequency sound waves by a transducer (electrical energy into sonic energy)
- sonic waves reflect toward the transducer
Anechoic tissues:
Free from echo
Don’t produce signals
Appear black
Fluid filled cyst
Hypo echoic tissues:
Produce a weak signal
Hyper echoic tissues:
Produce intense signals , ligaments , skin, needles, or catheters, appear bright
Which of the 6techniques are non ionising
MRI
Ultrasonography
Sialography:
- Radiographic examination of the salivary glands, involves injecting contrast medium into the salivary duct of a single gland
Indications of sialography:
- functional integrity of the salivary glands
- obstructions
- ductal patterns
- Intra glandular neoplasms
Sialography procedure :
- duct is dilated using lacrimal probes
- a cannula is inserted into the duct
- radio opaque fluid (iodine) is injected
- a series of radiographs are taken to determine the flow of the fluid, identify obstructions, and the rate of fluid excretion from the gland
