Computed TomographyComputed Tomography

Computed TomographyComputed Tomography• Introduced in 70’s• Principle: Internal structures of an

object can be reconstructed from multiple projections of the object

Philips CTVision Philips CTVision SecuraSecura

Mechanism of CTMechanism 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

DetectorsDetectors• Scintillation crystals• Xenon-gas ionization chamber

Scintillation CrystalsScintillation Crystals• Materials that produce light

(scintillate) when x-rays interact• Similar to intensifying screen• Number of light photons produced

energy ofincident x-ray beam• Light photons need to be

converted to electrical signal

Ionization ChamberIonization Chamber• X-ray ionizes

xenon gas• Electrons move

towards anode• Generates small

current• Converted to

electrical signal

AttenuationAttenuation• 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 - VoxelPixel - Voxel• Pixel - picture

element• Voxel - volume

element

CT NumberCT NumberTypical CT values

Tissues Range (Hounsfield unit)Air -1000Lung -200 to –500Fat -50 to –200Water 0Muscle +25 to +45Bone +200 to +1000

Image Display: WindowingImage 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 CTCone 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 Benefits of Cone Beam ImagingImaging• 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 Anatomic Landmarks on CTCT

Axial CT SectionsAxial CT Sections

1. Zygomatic Arch2. Lat. Pterygoid plate3. Optic canal4. Sphenoid sinus5. Soft tissues of

nasopharynx

Coronal Coronal SectionsSections

1. Frontal bone (orbital plate)

2. Ethmoid air cells3. Middle concha4. Maxillary sinus5. Inferior concha

1. Vomer2. Ramus3. Follicle of molar4. Gr. wing of

Sphenoid5. Tongue6. Mylohyoid m

Magnetic Resonance Magnetic Resonance ImagingImaging

Magnetic Resonance Magnetic Resonance ImagingImaging• Three steps of MRI• MRR

– Magnetic Field– Radio-frequency Pulse– Relaxation

Magnetic Moment Magnetic Moment DirectionDirection

Application of RF PulseApplication of RF Pulse

Relaxation

Spin or Angular MomentSpin or Angular Moment• 1H, 14N, 31P, 13C, and 23Na 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 MomentMagnetic 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 NucleusHydrogen Nucleus• Most abundant• Yields strongest MR signal

Radiofrequency PulseRadiofrequency Pulse• RF pulse is an electromagnetic

wave• Caused by a brief application of an

alternating electric current

Receiver CoilsReceiver 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 InteraPhilips Gyroscan Intera

RelaxationRelaxation• 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 RelaxationT1- 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 RelaxationT2- 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 StrengthsMagnetic 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

0.00005 Tesla (0.5 Gauss)