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CT and MRI
Terms in this set (53)
Invention of CT
1963 Cormack did preliminary work.
1970-1971 invention credited to Hounsfield (Electro musical instruments EMI) Beatles
1973 1st CT (head) took 9 days. June.
1979 - noble prize to Hounsfield and Cormack
Multi planer reconstruction. Ability to reconstruct axial images into coronal, sagittal, or oblique planes.
Only capable of scanning head. Used water bag to eliminate air interface.
C arm configuration - pencil slit X-ray beam, 1 degree increments, 180 projections, two detectors (sodium iodide scintillation crystals/photomultiplier tubes)
Fan beam. 30 degree increments. Linear detectors.
Wider fan beam and curved detectors. 360 degree rotation 1 time = one section. Prone to ring artifacts.
Rotating tube. Detector array.
Ring artifacts due to failure of one or more detector elements.
Stationary detectors in 360 degree array. Higher dose. Rotating tube/stationary detector.
Electron beam tomography (EBT) or cardiac Cine CT. High speed CT scanner
Helical/spiral (misnomer). Slipping technology. Ex bumper cars or metro train.
MSCT multi section helical CT. Multiple aligned row of detectors.
Sequential scanning vs helix scanning
Seq scan - one 360 degree scan per anatomical section. Non continuous to avoid cord tangling.
Helix scan - cord is attached to tube. Doesn't get tangled. Tube rotates around patient.
Pitch of .5 = more info more dose. Over sampling.
Pitch of 1 = perfect
Pitch of 2 = less info less dose.
Different routes of contrast
Oral, IV, rectally
Contrast media for CT
Must be more diluted than X-ray. Iodine based IV contrast. 2% barium mixture is used.
Iodinated oral contrast can be used (hypaque).
Gastrograpen must be at a low concentration to prevent contrast artifacts.
CT vs X-ray
CT better contrast resolution due to reduced scatter radiation
X-ray better spatial resolution
Gantry, table, X-ray tube, detectors, computer, display console, image storage units.
Circular device that houses the Data Acquisition system DAS. Better contrast than X-ray.
Tube, detectors, filters, collimator, ADC.
Much higher heat load. Ceramic target backing. 10,000 exposures in single month. Double collimation.
Scintillation crystals (sodium iodide) and photomultiplier tubes. Fast response time without lag. Quick signal unload.
Converts the measurement into an electrical signal proportional to the radiation intensity.
Direct or indirect capture??
Four basic functions - image display, control of data acquisition, image reconstruction, storage of image data.
Ct claim to fame
Exceptional contrast resolution due to the reduction in amount of scatter. No superimposition and 2 collimator.
X-ray has better spatial resolution.
Pixel and voxel
Pixel - 2 dimensional. Picture element
Vogel - 3 dimensional. Volume element
Hounsfield units or CT numbers
Each pixel within matrix is assigned a number that is related to the linear attenuation coefficient of tissue.
Air -1000, water 0, bone +1000
Window width and level
Window width - contrast. range of CT numbers that are used to map signals into shades of gray. Wide/narrow or long/short.
Window level - determines midpoint of range of gray levels to be displayed. Darker/lighter.
Magnetic resonance imaging (MRI)
Not ionizing radiation. Uses magnetic resonance (radio waves). Computer based cross sectional imaging modality. Similar to CT.
Normally protons point in random directions. In a magnetic field the nuclei attempts to line up with the direction of the magnetic field.
Body is 63% Hydrogen and 80-100% water.
Occurs at a specific frequency or rate due to magnetic force. Wobbling motion. spinning top.
Hydrogen line up with magnetic field - low energy state. "Lazy ones"
Hydrogen line up opposite of magnetic field - high energy state. "Fight" the mag field.
RF Energy. Used to irritate the low energy hydrogen.
The coil that is used to produce the energy to irritate the low energy hydrogen is called the RF coil.
The irritated low energy hydrogen absorbs the energy from RF coil and turns into high energy hydrogen.
It then releases the energy (wants to go back to being lazy) in the form of waves.
The energy waves are received by the receiver coils. Once the hydrogen releases the energy it goes back to being lazy.
Receiver coils converts the energy wave into electrical current signal and that's how the MRI machine is able to detect hydrogen.
Depends on signal to noise ratio (SNR). The closer the coils are to the patient the better the SNR.
Transmit receive coils, receive only coils, transmit only coils.
Energy is absorbed. Hydrogen goes from low energy to high energy. Hydrogen sync up and you get transverse magnetization.
Coils that modify the main magnetic field. 3 sections of coil. One for each plane. Stronger at one end and weaker on the other.
Changing the frequency emitted by RF coils, the MRI machine can look for hydrogen section by section.
Determine slice thickness, plane, resolution.
Nuclear density (proton density) - good at demonstrating basic anatomy.
Relaxation rates (T1 and T2) - good at identifying characterizes of different tissue types. T1 - dark. T2 - white
Flow phenomena (MRA)
Nuclear/proton density (PD or SD ((spin density))
Quantity of spins in tissue relative to proton density of water.
Transverse magnetization decay.
Time it takes for spins angled to B to lose 63% of their coherence.
No energy transfer.
Transfer of energy to lattice. Protons go back to low energy state.
Time it takes 63% of M to realign itself with B after RF excitation has been removed.
Computer converts signal into shades of gray. Strong signals are white. Weak signals are black.
Measurement of magnet
Gauss or tesla. 42.58 MHz/tesla
1 Tessa = 10,000 gauss
Are able to absorb energy if they are exposed to RF pulses.
If the RF and nuclear precession are in same frequency the photons will absorb energy which is called RESONANCE.
Instrument & equipment
Magnet room, equipment room, operators console, patient table, computer sub system.
Magnet room - coils, magnet, FDA approved up to 3 tesla.
Resistive - .6 tesla. Simple, large electromagnets consisting of coils of wire. Lots of heat.
Superconductive (most common) 1.5-3 Tesla. Electromagnets, wire loops are cooled w/liquid helium.
Permanent - .3 tesla. Constant field that doesn't require cooling or electricity. Very HEAVY.
Houses electronics necessary for transmitting radio wave pulse sea.
Receives and analyzes MRI signal
Houses all electronics and computers for image processing.
Copper, steel, aluminum.
Bad for image.
Pacemaker, 1st trimester not recommended, zoning, retained bullets.
ear implant, IUD, contrast, hardware, credit cards, claustrophobia.
1) entrance to facility, 2) patient dressing room, 3) control room, 4) in room w/magnet.
Contrast used for MRI. Reduces relaxation time T1 & T2.
Reduces motion in bowel.
MRI vs CT
Gantry in MRI has more tubing.
MRI bone dark gray low signal., white strong signal.
Periodic vs aperiodic
Periodic - consistent motion (breathing, heart rate)
Aperiodic - inconsistent (peristalsis, digestion)
Release of energy by the excited protons is
All of the following are types of MRI magnets except
Resistive, superconductive, permanent are all magnets.
Which atom is most often imaged in MRI
Magnetic resonance signals contain info about which of the following tissue properties?
Proton density, relaxation rates, flow phenomena
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