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X-Ray Detection

X-Ray Detection. Radiography. Few high-quality images are made in a study Orthopedic Chest Abdomen (Mammography). Radiation Units . EXPOSURE Amount of radiation in an x-ray beam capable of ionizing air Total charge of electrons liberated per unit mass by x-ray photons

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X-Ray Detection

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  1. X-Ray Detection

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

  3. Radiation Units • EXPOSURE • Amount of radiation in an x-ray beam capable of ionizing air • Total charge of electrons liberated per unit mass by x-ray photons • Coulombs per kg (C/kg) or Roentgens (R ) • 1R =2.58 10-4 C/kg • Defined only for photons of E <3MeV

  4. 1 Rad = 100 erg/g = 0.001 J/kg = 0.001 Gray [Gy] [SI units] Absorbed Dose • Amount of energy (E) absorbed per unit mass (M) • D=E/M • SI units Grays • Depends on material so material and locations should always be specified • Radiation risk is related to absorbed dose 1erg=1e-7 joule

  5. Effective dose equivalent Effective dose equivalent HE[Sv] (Sievert) takes into account sensitivity of organ exposed: f factor – Conversion factor between exposure (X) and absorbed dose D = f X air, muscle, soft tissue f~ 1 bone f ~4 i: indicates organ w: relative organ sensitivity to radiation QF: Quality Factor = danger of type of radiation QF(x-ray, gamma) = 1)

  6. 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 • Exposed area: The larger the exposed area the greater the damage (collimators, shields!) • 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.]

  7. Analog X-ray imaging • Film • Intensifying screens • Scatter removal • Image intensifiers

  8. 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/Io) • D> 2 = "black", D = 0.25 … 0.3 = "transparent (white)" with standard light box (diagnostic useful range ~ 0.5 - 2.5)

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

  10. Film blackening Blackening is related to the number of photons reaching the film Measured in OD OD=-log (I/Io) The eye has a log response to ligth

  11. 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

  12. Intensifying 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 • One x-ray photon can generate multiple optical photons 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% (calcium tungstate) • Rare earth phosphors: LaOBr:Tb, Gd2O2S:Tb

  13. 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

  14. 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

  15. 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

  16. Applications

  17. Fluoroscopy • Allow real time observation of dynamic activities: barium moving through the GI tract • Low dose xray

  18. 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.

  19. 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.

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

  21. Anti-scatter grid • Significant Compton interaction for low Ep (37-50% of all photons). • Linear grid: Scatter-to-primary (SPR) reduction factor ~5 • Recently crossed grid introduced • Grids are moved during exposure • Longer exposure breast leadsepta detector

  22. 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

  23. X-ray phase contrast

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