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)