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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

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Biomedical Imaging I

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  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

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