1 / 31

Biomedical Imaging I

Biomedical Imaging I. Class 2 – X-Ray Imaging II: Instrumentation and Applications 09/21/05. X-Ray Generation. X-ray tube. Working Principle: Accelerated charge causes EM radiation: Cathode filament C is electrically heated ( V C = ~10V / I f = ~5 A) to boil off electrons

trixie
Download Presentation

Biomedical Imaging I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Biomedical Imaging I Class 2 – X-Ray Imaging II: Instrumentation and Applications 09/21/05

  2. X-Ray Generation

  3. X-ray tube • Working Principle: Accelerated charge causes EM radiation: • Cathode filament C is electrically heated (VC = ~10V / If = ~5 A) to boil off electrons • Electrons are accelerated toward the anode target (A) by applied high-voltage (Vtube = 40 – 150 kV); kinetic electron energy: Ke = e  HVusually rated in “peak-kilo voltage” kVp • Typical: Vtube = 40 – 150 kVp, Itube = 1-1000mA • Deceleration of electrons on target creates "Bremsstrahlung" VC, If + A - C kVp, Itube - +

  4. hn K Nucleus K’ Bremsstrahlung • Continuous spectrum of EM radiation is produced by abrupt deceleration of charged particles (“Bremsstrahlung” is German for “braking radiation”). • Deceleration is caused by deflection of electrons in the Coulomb field of the nuclei • Most of the energy is converted into heat, ~0.5 % is x-ray • The energy of the generated x-ray photon is given by energy conservation: • The maximum energy for the produced photon is given by:

  5. Bremsstrahlung intensity • Overall Bremsstrahlung intensity I: • The produced x-ray power Px (in[W]) is given by:Material constant k = 1.1×10-9 for Tungsten (Z=74). Electrical power consumption of tube: Ptube = Itube Vtube [W]

  6. Bremsstrahlung spectrum • Theoretically, bremsstrahlung from a thick target creates a continuous spectrum from E = 0 to Emax with intensity Ib 1/E :Ib(E) ~ Z(Emax-E) • Actual spectrum deviates from ideal form due to • Absorption in window / gas envelope material and absorption in anode • Multienergetic electron beam I Ep= hn Ep,max,layer 1

  7. Continuum E [keV] hn 0 0.5 N M 3 - - - Kg - - L 11 - a b - - K M Kb L L-lines - - Ka - - 70 K a b g K-lines Characteristic radiation • Narrow lines of intense x-ray at characteristic energies are superimposed on the continuous bremsstrahlung spectrum. • Caused by removal of inner shell electrons and subsequent filling of hole with electrons from higher shell under emission of x-ray at shell-energy difference • Lines are named after the lower shell involved in the process; the upper shell involved is denoted by Greek letters: Dn = 1 a-transitions, Dn = 2 b-transitions, ...

  8. 74W X-ray spectra • X-ray for general diagnostic radiology produced at 40 – 150 kVp • Maximum photon energy: Ep[keV] = hnmax =e  kVp • Characteristic radiation occurs only for anode voltages e kVp > IK,L,M,…

  9. X-ray tube design • Cathode w/ focusing cup, 2 filaments (different spot sizes) • Anode • Tungsten, Zw= 74, Tmelt=2250 ºC • Embedded in copper for heat dissipation • Angled (see next slide) • Rotating to divert heat

  10. Reduction of anode heating • Anode angle of 7º…15º results in apparent or effective spot size Seffectivemuch smaller than the actual focal spot of the electron beam (by factor ~10) • Seffective depending on image location • Rotation speed ~ 3000 rpm • Decreases surface area for heat dissipation from w  (r2 - r1) to p(r22- r12); generally by a factor of 18-35.

  11. Limitations of anode angle • Restricting target coverage for given source-to-image distance (SID) • "Heel effect" causes inhomogeneous x-ray exposure

  12. Magnification and image blur • Geometric magnification given by • Reduction of M by minimizing B, i.e. placing patient next to film. Finite target thickness can lead to variations in M. • Blurring of edges and fine structures due to finite source size leads to penumbrap: loss of spatial resolution

  13. 1 Rad = 100 erg/g = 0.001 J/kg = 0.001 Gray [Gy] [SI units] X-ray dosimetry I • The radiation absorbed doseD [Rad] is defined as • Effective dose equivalent HE[Sv](Sievert) takes into account sensitivity of organ exposed: i: indicates organ w: relative organ sensitivity to radiation QF: Quality Factor = danger of type of radiation QF(x-ray, gamma) = 1)

  14. Biological effects of ionizing radiation • Damage depends on deposited (= absorbed) energy (intensity  time) per tissue volume • Threshold: No minimum level is known, below which damage occurs • Exposure time: Because of recovery, a given dose is less harmful if divided • Exposed area: The larger the exposed area the greater the damage (collimators, shields!) • Variation in Species / Individuals: LD 50/30 (lethal for 50% of a population over 30 days, humans ~450 rads / whole body irradiation) • Variation in cell sensitivity: Most sensitive are nonspecialized, rapidly dividing cells (Most sensitive: White blood cells, red blood cells, epithelial cells. Less sensitive: Muscle, nerve cells) • Short/long term effects: Short term effects for unusually large (> 100 rad) doses (nausea, vomiting, fever, shock, death); long term effects (carcinogenic/genetic effects) even for diagnostic levels  maximum allowable dose 5 R/yr and 0.2 R/working day [Nat. Counc. on Rad. Prot. and Meas.]

  15. X-Ray Detection

  16. Radiography • Few high-quality images are made in a study • Orthopedic • Chest • Abdomen • (Mammography)

  17. Photographic film • Photographic film has low sensitivity for x-rays directly; a fluorescent screen (phosphor) is used to convert x-ray to light, which exposes film • Film Composition: • Transparent plastic substrate (acetate, polyester) • Both sides coated with light-sensitive emulsion (gelatin, silver halide crystals 0.1-1 mm). Exposure to light splits ions  atomic silver appears black (negative film) • Blackening depending on deposited energy (E= I  t) • Optical density (measure of film blackness) for visible light: D = log (Iincident/Itransmitted) • D> 2 = "black", D = 0.25 … 0.3 = "transparent (white)" with standard light box (diagnostic useful range ~ 0.5 - 2.5)

  18. XD1 Film characteristic curve (H and D curve) I • Relationship between film exposure and optical density D • Film characteristics: • Fog: D for zero exposure • Sensitivity (speed S): Reciprocal of exposure XD1 [R] that produces D of one: • Linear region S=1/XD1

  19. Film characteristic curve II • Film characteristics continued: • Film gamma g (maximum slope): • Contrast C=DD/DlogX • Latitude: Range of exposure causing appreciable values of D 0.5…2.5 • Compromise between maximum gradient and latitude causes under- / overexposed regions in the image. Contrast, latitude Film gamma

  20. Film sensitivity & resolution • Tradeoff between sensitivity (S) and resolution (R): • Grain size: coarse: S / R fine: S / R • Coating thickness: thick: S / R thin: S / R • No. of coatings: dual: S / R single: S / R

  21. Fluorescent screens • Scintillators ("phosphors") are used to convert x-ray energy to visible or near-infrared light through fluorescence • The light intensity emitted by screen is linearly dependent on x-ray intensity • Because Ep,x-ray(100 … 10,000) Ep,vis one x-ray photon can generate multiple optical photons • Quantum detection efficiency (QDE): Fraction of incident x-rays that interact with screen (30-60%). • Conversion efficiency: Fraction of the absorbed x-ray energy converted to light. • CaWO4: 5% • Rare earth phosphors: LaOBr:Tb, Gd2O2S:Tb, Y2O2S:Tb: 12 - 18%

  22. Screen / Film Combinations • Sandwiching phosphor and film in light tight cassette. • Lateral light spread through optical diffusion limits resolution, can be minimized by absorbing dyes • Screen thickness is tradeoff between sensitivity and resolution X-ray photons X ray Film emulsion Crystals Light-tight cassette Phosphor screen Foam Film Light spread

  23. cassette photoreflective layer fluorescent screen photosensitive layer film substrate Characteristics of Fluorescent Screen • Fluorescence wavelengths are chosen to match spectral sensitivity of film:CaWO2: 350nm-580nm, peak @ 430 nm (blue)Rare earths: Gd: green La: blue • Dual-coated film, two screen layers • Optically reflective layers

  24. Fluoroscopy • Lower x-ray levels are produced continuously and many images must be presented almost immediately • Angiography • GI tract

  25. Image Intensifier • Image intensifier tubes convert the x-ray image into a small bright optical image, which can then be recorded using a TV camera. • Conversion of x-ray energy to light in the phosphor screen (CsI) • Emission of low-energy electrons by photomissive layer (antimony) • Acceleration (to enhance brightness) and focusing of electrons on output phosphor screen (ZnCdS) • Quantum detection efficiencies ~60% - 70% @ 59 keV

  26. Applications

  27. Mammography • Detection and diagnosis (symptomatic and screening) of breast cancer • Pre-surgical localization of suspicious areas • Guidance of needle biopsies. • Breast cancer is detected on the basis of four types of signs on the mammogram: • Characteristic morphology of a tumor mass • Presentation of mineral deposits called microcalcifications • Architectural distortions of normal tissue patterns • Asymmetry between corresponding regions of images on the left and right breast •  Need for good image contrast of various tissue types. • Simple x-ray shadowgram from a quasi-point source.

  28. Mammography contrast • Image contrast is due to varying linear attenuation coefficient of different types of tissue in the breast (adipose tissue (fat), fibroglandular, tumor). • Contrast decreases toward higher energies  the recommended optimum for mammography is in the region 18 - 23 keV depending on tissue thickness and composition.

  29. Mammography source • Voltage ~ 25-30 kVp • Target material Mo, Rh (characteristic peaks) • Filtering: Target Rh, Filter Rh Target Mo, Filter Mo

  30. Anti-scatter grid • Significant Compton interaction for low Ep (37-50% of all photons). • Linear grid: Lead septa + interspace material. Septa focused toward source. Grid ratio ~ 3.5-5:1. Only scatter correction in one dimension. Scatter-to-primary (SPR) reduction factor ~5 • Recently crossed grid introduced • Grids are moved during exposure • Longer exposure breast leadsepta detector

  31. X-ray projection angiography • Imaging the circulatory system. Contrast agent: Iodine (Z=53) compound; maximum iodine concentration ~ 350 mg/cm3 • Monitoring of therapeutic manipulations (angioplasty, atherectomy, intraluminal stents, catheter placement). • Short intense x-ray pulses to produce clear images of moving vessels. Pulse duration: 5-10 ms for cardiac studies …100-200 ms for cerebral studies

More Related