Basic Concepts of Other Imaging Modalities Dent 5101

Body-section Radiography A special radiographic technique that blurs out the shadows of superimposed structures Object of interest less blurred Does not improve the sharpness

Tube and Film Move in Opposite Direction

Tube and film move in opposite direction, and rotate about a fulcrum The level of the fulcrum is the focal plain

Blurring Blurring Determined by: –Distance of the tube travel –Distance from the focal plain –Distance from the film –Orientation of tube travel

Panoramic Radiography

Obtained by rotating a narrow beam of radiation in the horizontal plane The film is rotated in the opposite direction while the object (jaws) is stationary

Focal Trough A 3-dimensional curved zone or image layer in which structures are reasonably well defined.

Types of Panoramic Machines Panorex – Two centers of rotation. Interruption of exposure in the midline Orthopantomogram – Three centers of rotation. Continuous image

Panorex Image

Orthopantpmograph

Image Intensification

Early Fluoroscopy Early fluoroscopy done by direct observation Screen was poorly illuminated - image perception inadequate

Image Intensification Image intensifier improved viewing of fluoroscopy

Intensifier Tube Four parts: –Input phosphor and photocathode –Electrostatic focusing lens –Accelerating anode –Output phosphor

Intensifier Tube (Cont.) Input phosphor: cesium iodide (CsI) or zinc- cadmium-sulfide. Photocathode: A photo-emissive metal. Electrostatic focusing lens: series of negatively charged electrodes—focuses the electron beam. Output phosphor: Provides thousand-fold more light photons.

Intensifier Tube Used in: –Sialography –Arthrography

Computed Tomography

Introduced in 70’s Principle: Internal structures of an object can be reconstructed from multiple projections of the object

Philips CTVision Secura

Mechanism of CT X-ray tube is rotated around the patient Radiation transmitted through the patient is absorbed by a ring of detectors Absorbed radiation is converted to an image Detectors

Detectors Scintillation crystals Xenon-gas ionization chamber

Scintillation Crystals Materials that produce light (scintillate) when x-rays interact Similar to intensifying screen Number of light photons produced energy ofincident x-ray beam Light photons need to be converted to electrical signal

Ionization Chamber X-ray ionizes xenon gas Electrons move towards anode Generates small current Converted to electrical signal

Attenuation Reduction in the intensity of an x- ray beam as it traverses matter, by either the absorption or deflection of photons from the beam

Pixel - Voxel Pixel - picture element Voxel - volume element

CT Number Typical CT values TissuesRange (Hounsfield unit) Air-1000 Lung-200 to –500 Fat-50 to –200 Water0 Muscle+25 to +45 Bone+200 to +1000

Image Display: Windowing Usual CRT can display ~256 gray levels 2000 CT numbers Select the CT number of the tissue of interest, then range of ±128 shades

Cone Beam CT Uses cone shaped x- ray beam. Beam scans the head in 360 degrees. Raw data are reformatted to make images

Benefits of Cone Beam Imaging Less radiation than multi-detector CT due to focused X-rays (less scatter) Fast and comfortable for the patient (9 to 60s) Procedure specific to head and neck applications One scan yields multiple 2D and 3D images

Anatomic Landmarks on CT

Axial CT Sections

1.Zygomatic Arch 2.Lat. Pterygoid plate 3.Optic canal 4.Sphenoid sinus 5.Soft tissues of nasopharynx Coronal Sections

1.Frontal bone (orbital plate) 2.Ethmoid air cells 3.Middle concha 4.Maxillary sinus 5.Inferior concha

1.Vomer 2.Ramus 3.Follicle of molar 4.Gr. wing of Sphenoid 5.Tongue 6.Mylohyoid m

Magnetic Resonance Imaging

Three steps of MRI MRR –Magnetic Field –Radio-frequency Pulse –Relaxation

Magnetic Moment Direction

Application of RF Pulse Relaxation

Spin or Angular Moment 1 H, 14 N, 31 P, 13 C, and 23 Na has nuclear spin They spin around their axes similar to earth spinning around its axis Elements with nuclear spin has odd number of protons, neutrons

Magnetic Moment When a nucleus spins, it has angular momentum When the spinning nucleus has a charge, it has magnetic dipole moment Moving charges produce magnetic fields

Hydrogen Nucleus Most abundant Yields strongest MR signal

Radiofrequency Pulse RF pulse is an electromagnetic wave Caused by a brief application of an alternating electric current

Receiver Coils Send or “broadcast” the RF pulse Receive or “pick up” the MR signals Types: Body coils, head coils, and a variety of surface coils

Philips Gyroscan Intera

Relaxation This is the process that occurs after terminating the RF pulse The physical changes caused by the RF pulse revert back to original state

T1- Spin Lattice Relaxation At the end of RF pulse, transversely aligned nuclei tend to return back to equilibrium This return to equilibrium results in the transfer of energy

T2- Spin-spin Relaxation While the nuclei are in transverse phase, their magnetization interfere with each other. This interference leads to the loss of transverse magnetization.

Magnetic Field Strengths Measured in Tesla or Gauss Usual MRI field strength ranges from 0.5 to 2.0 Tesla Earth’s magnetic field is about Tesla (0.5 Gauss)

Advantages of MRI Higher resolution of tissues No ionizing radiation Multiplanar imaging

Disadvantages of MRI Long imaging time Hazards with ferromagnetic metals (pacemakers, vascular clips, etc) Claustrophobia Higher cost

Relative Brightness of Tissues FatWhite Marrow Brain MuscleGray Body Fluid TMJ Disk Cortical Bone AirBlack

Nuclear Medicine

Radioactive compounds Target tissues Radioactive agents pools in the target tissues Detected and imaged by external detectors (gamma camera).

Nuclear Medicine Shows structure and function of the target tissues Static and dynamic conditions Scintigraphy scans or RN (radionuclide) scans Bone scans or salivary gland scans

Technetium 99m TcO thyroid and salivary gland scan 99 Tc phosphate - bone scan

Is this an active disease?

Phases of Salivary Gland Scan Flow phase: –Five to 10 mCi of 99m TcO 4 –first 30 to 120 seconds –shows flow of blood Concentration phase: –next 30 to 45 minutes –demonstrate the anatomy and function Washout Phase: –administer sialagogue –demonstrates secretory capabilities

Cephalometric Radiography Reproducible and standardized views For measurements and assess growth Fixed source to film distance – 60 inches Cephalostats and earplugs help in reproducible positions

Cephalometric Radiography

Contrast Agents

Radiopaque materials Water soluble Fat soluble 28 – 38% iodine

Phases of Sialography Ductal Acinar Evacuation

Indications of Sialography Acute swelling secondary to ductal obstruction Recurrent Inflammation Palpable salivary gland mass Autoimmune Sialadenitis

Contraindications of Sialography Sensitivity to contrast agents Acute Sialadenitis Limited use in tumor diagnosis

Scintigraphy Sialography Radioactive materialRadiopaque material Through blood stream Through duct All glands imaged at the same time One gland at a time Imaged by gamma camera Imaged by fluoroscopy

Contrast Studies: Arthrography

Arthrography Contrast media is introduced in joint spaces Upper vs. lower joint space Viewed by Image Intensifier Fluoroscopy Video recording allows study of joint movement

Contrast Material Injection

Open Position Translation of condyle Reduction of disk