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History of medical imaging. Roentgen : Discovered x-rays Nov 8, 1895 at his laboratory in Wurzburg, Germany. Submitted for publication on Dec. 28, 1895 Published Jan. 5, 1896 First clinical image was a needle in a woman's hand. Used to guide surgeons. Jan. 13, 1896 Nobel prize in 1901 Miller : First x-ray imaging in US. Hounsfield : Invented Computed Tomography (CT) Lauterbur : Nobel Prize in MR Imaging, CWRU Grad Energy and radiation. Units : Dose versus exposure : If we record a measurement with an air chamber or other radiation measurement device, we measure exposure. Only if we determine the amount absorbed in a tissue should we speak about dose. X-ray production.
Characteristic radiation : Anode materials: Most common: Tungsten (W, Z = 74) In mammography: Molybdenum (M, Z= 42) and Rhodium (Rh, Z = 45) X-ray tube spectra: Filtration:
- E must be greater than binding energy to get a photoelectric absorption event
- Probability of event ~ Z^3 /E^3
- ~ Z^3 -> differential absorption and contrast in images
- ~ 1/E^3 -> more image contrast at low energy
- Discontinuities in attenuation curve due to increased probability of interaction just above the binding energy
- K-edge absorption makes iodine a good contrast agent
- This absorption event is useful for creating image contrast and good image quality. Compton scatter:
- Scattering, not an absorption event
- Interaction between x-ray and outer shell electron with low binding energy
- Scattered photons have sufficient energy to travel to detector -> degrade image
- Scattered photons can deliver dose to personnel
- Probability proportional to number of electrons per volume which is fairly constant in tissue
- Probability fairly independent of x-ray energy
- With regard to image quality, Compton scattering is the bad interaction. Effect of scatter on the final image: X-ray physics, scatter and contrast. Derivation of μ from atomic properties: Electron Density = [Avogadros #] * [Atomic #] / [Atomic mass] Square law distance effect: Magnification:
Focal spot blurring: Purpose/function of grids and Grid Ratio: Subject contrast : Image quality. Poisson statistics: MTF: A narrow PSF makes a wide MTF and a wide PSF makes a narrow PSF
Automatic exposure controls: DSA and Processing: Computed tomography. Hounsfield units: Window/ level: Partial volume: Fourier slice theorem: Convolution back projection:
ART:
Noise: Beam hardening artifact: 3D Medical Image Visualization Ambient : Indirect light that is reflected off other objects. Diffuse : Light that comes from a light source and is spread equally in all directions. Independent of the observer. Specular : Light at comes from a light source and is directly reflected from a surface. Depends on viewer and source.
ROC :
Ultrasound Compression wave:
Velocity of sound: Attenuation: Wave equation: Reflection: Noise: Speckle Doppler shift:
- When the pulse is turned off, the transverse vector will decay, and the longitudinal vector will regrow. The rate the transverse vector decays is known as the Free Induction Decay (FID)
- This response (the decaying of the Transverse Vector) is known as T2 relaxation or “spin-spin” interaction. This is because it is based on the proton to proton spin interactions with each other.
- The regrowth of the Longitudinal Vector is known as the T1 relaxation or the ”spin-lattice” interaction. This is because the interaction is based on the spin interactions with the surround tissue structures. T1, T2, proton density: Proton Density Weighted:
- Because singular protons are most abundant in water and fats, this weighting is mostly a measure of water and fat density. T1-Weighted Image: T2-Weighted Image:
Mechanisms of image contrast: Slice selection: Spin echo sequence: MRI image reconstruction:
- Due to B 0 homogeneity, there will be no inherent difference in received signals that are precessing at the Larmour Frequency. If two objects were in a Magnetic field (B 0 ), their resultant signals (M 1 + M 0 ) would simply combine with no discernable, discrete difference in signals.
- Introducing a gradient allows two discrete signals to be received and interpreted to discern the two signals (M 1 + M 2 ) Radionuclide imaging Radioactivity and decay mathematics: Radioactive Decay:
ln ( 2 )
T 1
2
A (^ t )= A 0 e
− λt At: t = T1/2, e-λt^ = 0.
A
T 1
=0.5 A 0
Radiopharmaceuticals:
Molecular Imaging PET and MR techniques: Reporter genes and probes Bioluminescent imaging Fluorescence imaging Radionuclide imaging with reporter genes; e.g., HSV1-tk OCT imaging
