Projection Radiography

Transcription

1 Medical Physics Projection Radiography Ho Kyung Kim Pusan National University Projection radiography Most commonly used method of medical imaging utilizing x rays, called conventional radiography Representing a projection of the 3-D volume of the body onto a 2-D imaging surface Representing the transmission of the x-ray beam thru the patient, weighted by the integrated loss of beam E due to scattering & absorption in the body Regarding shadow cast by a semitransparent body illuminated by x rays limiting depth resolution; hence contrast To screen for pneumonia, heart disease, lung disease, bone fractures, cancer & vascular disease Overall imaging conditions of projection radiographic system Exposure time ~0.1 sec Image size in. Exposure level 30 mr of a chest radiograph (= 1/10 of the annual bgn dose) 2 1

2 General projection radiography system Fluoroscopy system (C-arm system) Mammography system 3 Instrumentation X-ray tube Filters/collimators Grids Detectors 4 2

3 X-ray tubes 5 Cathode assembly Filament Thin thoriated* tungsten wire (*thoriate: add thorium or thorium dioxide to increase thermionic flow) Typ. filament current = 3 5 A at 6 12 V Thermionic emission of e s, accelerated toward the anode producing tube current, referred to as the ma (typ. 50 1,200 ma) Filament current heat (resistance) # of discharged e s tube current Focusing cup Small depression in the cathode containing the filament To help focus the e beam toward a particular spot on the anode Focal point: a bevelled edge of the anode disk 6 3

4 Anode Made from molybdenum on which a rhenium-alloyed tungsten target is coated Mo in the target area for mammography x-ray tube Tube voltage kvp (typ kvp) Generating characteristic & bremsstrahlung x rays ~1% of conversion efficiency (~99% heat) Stator electromagnets induce the rotors to rotate the anode To avoid melting the anode target area 3,200 3,600 rpm 7 Exposure ma (tube current) s (exposure time) = mas Controlled by fixed timer circuit or automatic exposure control (AEC) Controlling the overall exposure determined by the duration of the applied kvp Fixed timer circuit Silicon-controlled rectifier (SCR) switch timed by a microprocessor Timing accuracy ~ s Radiologists control the ma & the exposure time directly, hence determining the mas AEC timer 5-mm-thick parallel plate ionization chamber placed btwn the patient table & the imager Voltage signal from ion chamber triggers the SCR, which shuts off the tube voltage Radiologists control the mas & the exposure time is determined automatically by the AEC Set max. time to prevent accidental overdose due to that AEC circuit malfunctions or the ion chamber is missing or incorrectly positioned 8 4

5 Filtration & restriction Filtration Process of absorbing low-e x-ray photons before they enter the patient X-ray spectrum: polyenergetic, bremsstrahlung + characteristic radiations Very undesirable for low-e photons to enter the body Almost entirely absorbed w/i the body contributing to patient dose but not the image Inherent filtering Tungsten anode itself Glass housing of the x-ray tube & the dielectric oil Added filtering Placing metal in the x-ray beam outside of the tube Al (1 2 mm thick) typically Higher-E systems: Cu + Al (to attenuate the 8 kev characteristic x rays from Cu) Additional 1.0 mm Al/Eq of filtering by the silvered mirror placed w/i the collimator Beam hardening The increase in the beam s effective energy as a progressive shift in the position of the spectrum to the right due to filtering Note that the National Council on Radiation Protection & Measurements (NCRP) recommends a min. filtration of 2.5 mm Al/Eq at the exit port of the x-ray tube for the reduction of patient dose

6 Restriction beam Process of absorbing the x rays outside a certain field of view To avoid exposing parts of the patient that need not be imaged To help reduce the deleterious effects of Compton scatter Basic kinds of beam restrictors 1 Diaphragms Flat pieces of lead w/ holes Simple & inexpensive Fixed geometry used in dedicated systems that have only on purpose (e.g., chest imaging) 2 Cones or cylinders 3 Collimators Variable diaphragms comprised of movable pieces of lead Expensive but flexible Usually two collimators: one near the tube & one farther away from the tube» Mirror btwn them to illuminate the FOV w/ an alignment grid

7 Compensation filters & contrast agents Attenuation: the process by which x rays are absorbed or redirected (scattered) w/i the body or other objects in the FOV Different amounts of attenuation in body tissues ~, E differential attenuation contrast Sometimes need to artificially change the natural attenuation of the body prior to detecting x rays Compensation filters Used when difficult to obtain images of subjects having big differences in attenuation over area because of the limited dynamic range of x-ray detectors Specially shaped Al or leaded-plastic object for an x-ray detector w/ smaller dynamic range Placed btwn the x-ray source & the patient or in some cases btwn the patient & the detector

8 Contrast agents Used when difficult to visualize different soft tissues due to insufficient intrinsic contrast Chemical compounds to increase x-ray absorption w/i an anatomical region Utilizing the K-edge absorption effect Iodine (Z = 53): E K = 33.2 kev Intravascular injection or ingestion Blood vessels, heart chambers, tumors, infections, kidneys, bladder Barium (Z = 56) : E K = 37.4 kev Administered as a chalky milkshake Gastrointestinal tract

9 Grids, airgaps, & scanning slits Scattering process Causing a random fog throughout the image Reducing the contrast of image Reduction methods: grids, airgaps, & scanning slits Grids Thin strips of Pb alternating w/ highly transmissive interspace mat l (e.g., Al or plastic) Linear, focused grid 17 Crosshatch grids, parallel grids, Grid ratio A measure of the effectiveness of the grid for reducing scatter grid ratio = h b h = height of the lead strips b = spacing btwn the lead strips 6:1 16:1 in conventional radiographic systems Down to 2:1 in mammography systems Grid spacing Generally reported using its reciprocal, known as grid frequency 60 cm -1 for conventional radiographic systems 80 cm -1 for mammography systems Note that grids w/ higher grid ratios (tall lead strips or fine lead strip spacing) are more capable of stopping off-axis radiation 18 9

10 Consider the tradeoff btwn primary & scatter radiations Grid conversion factor (GCF) Characterizing the amount of additional exposure required for a particular grid mas with the gird GCF = mas without the grid Ranging from 3 to 8 Use a grid when the tube voltage > 60 kvp (isotropic scattering for low-e photons); when imaging a body part thicker than 10 cm Stationary grids Introducing visible artifacts Potter-Bucky grids Moving 2 to 3 cm during exposure in a linear or circular path 19 Airgaps Leaving an airgap btwn the patient & the detector an effective means of scatter rejection Increasing geometric magnification & blurring or unsharpness Scanning slits Placed in front of and/or in back of the patient Providing greater than 95% scatter reduction More complex & costly system Longer exposure times 20 10

11 Film-screen detectors Direct exposure of x-rays to a photographic film About 1 to 2% of the x rays are stopped by the film ( detection efficiency) Requiring an unnecessarily large x-ray dose to the patient A very inefficient way to create a photograph Use of intensifying screens on both sides of the film, but an additional image blurring due to light scattering should be considered 21 Intensifying screens Base Provided for mechanical stability Somewhat flexible to be pushed tightly against the film Typ. made of polyester plastic Reflective layer Reflecting light from the phosphor back into the film rather than getting lost in the base Typ. about 25 m thick Made of magnesium oxide (MgO) or titanium dioxide (TiO 2 ) Protective coating Applied to film side of the screen to protect it from repeated film loading & unloading 22 11

12 Phosphors Converting x rays into light Luminescent materials Fluorescence s of the excitation Phosphorescence in which light emission can be delayed Causing "afterglow" motion artifact, image lag Should be highly x-ray attenuating high Z (so large ) Should emit many light photons for every x-ray photon that is stopped High conversion efficiency» A measure of the number of light photons emitted per incident x-ray photon» Typ. btwn 5 to 20% depending on the type of phosphor & its thickness» ~10 3 light photons per incident 50-keV x-ray photon» Speed of screen "Faster" if the conversion efficiency is higher Calcium tungstate (CaWO 4 ) discovered by Thomas Edison Rare earth phosphors in the late 1970s 23 Radiographic film Optical film to capture the optical image created w/i the screens that sandwich the film Common size in US: 14 17, 14 14, 10 12, 8 10, & 7 17 in. Image quality depends on optical properties & details of chemical development Radiographic cassette A holder for two intensifying screens & the film "sandwiched" btwn One side "radiolucent"; the other "a sheet of lead foil" 24 12

13 X-ray image intensifiers XRII in fluoroscopy requires low-dose, real-time projection radiography 25 Input window Aluminum or titanium w/ mm thickness minimal loss of x-ray photons but capable of holding a vacuum Input phosphor Typ. CsI(Na) w/ 0.5 mm thickness cm in diameter 0.5-mm-thick aluminum reflector Photocathode Generating free e s w/i the vacuum tube Shaping e s into an (inverted) e intensity image Alternating voltage profile to provide variable image magnification Dynodes A series of electrodes to accelerate e s Anode kv relative to the cathode 26 13

14 Output phosphor screen P20-type phosphor Deposited on the output window (15-mm-thick glass & mm in diameter) Thin Al film on the inner side of the phosphor as the anode & reflector Lens Magnifying images Light sensor Providing an automatic gain control (i.e., feedback to the x-ray tube current) to provide a relatively constant image brightness TV camera Capturing the light image & sending to a TV monitor for real-time views Film camera Capturing selected still images onto film 27 Image Formation Basic imaging equation Consider a particular line segment thru the object starting at the x-ray origin & ending on the detector plane at point (x, y): Intensity of x rays incident at (x, y) E max I(x, y) = S0 E E e (x,y) 0 μ s;e ;x,y ds de 0 S 0 E = the spectrum of the incident x-ray s = distance from x-ray origin to the point (x, y) on the detector plane = r 28 14

15 Geometric effects Undesirable ( multiplicative ) effects due to the natural x-ray beam divergence Inverse square law Dependency of the net flux of photons (i.e., photons per unit area) on 1/r 2 Intensity at the origin of the detector assuming no attenuation I 0 = I S 4πd2 [# of photons/area] I S = beam intensity [# of photons] d = the source-to-detector distance Intensity at r = r(x, y) on the detector I r = I S 4πr 2 cos 2 drop-off of x-ray intensity due to the inverse square law I r = I S 4πr 2 = 4πd2 I 0 4πr 2 = I d 2 0 r 2 = I 0cos 2 θ Resulting in false object attenuation in a circular pattern around the detector origin 29 Obliquity Decreasing the beam intensity away from the detector origin when the detector surface is not orthogonal to the direction of x-ray propagation Implying that x rays pass through a larger area on the detector lowering x-ray flux resulting in a lower measured x-ray intensity on the detector surface I d = I 0 cos θ oblique area A d = A, where A = the cos θ area orthogonal to the beam direction 30 15

16 Beam divergence & flat detector Reduction in beam intensity due to 1) The inverse square law effect 2) Obliquity I d = I 0 cos 3 θ Anode heel effect Nonuniform x-ray intensity distribution along the cathode-anode direction, but no variation orthogonal to the cathode-anode direction Stronger intensity in the cathode direction ~45% intensity variation in the cathode-to-anode direction Far outweighing the effects of obliquity & inverse square law (i.e., more important) Compensated by an x-ray filter that is thicker in the cathode direction 31 Path length X-ray intensity at (x, y) = (0, 0): I d 0,0 = I 0 e μl X-ray intensity at (x, y) (0, 0): I d x, y = I 0 e μl cos θ Considering the 1/r 2 & obliquity: I d x, y = I 0 cos 3 θ e μl cos θ Causing a shading artifact w/i a homogeneous (attenuation & thickness) object Misleading as a different attenuation w/i the object or different object thickness 32 16

17 Depth-dependent magnification Consider the stick object of height w: w z on the detector is different depending on the position z of the object in FOV w z = w d magnification M z = d z z Different sizes for two object w/ the same size ( judgment about relative sizes of anatomical features must be made) Longitudinal study at the same radiographic conditions & patient positioning Depth-dependent blurring 33 Imaging equation w/ geometric effects Consider an idealized object t z (x, y): Infinitesimally thin; Located in a single plane z; Capable of differentially attenuating x rays as a function of x & y; Can be regarded as a transmittivity I d x, y = I 0 cos 3 θ e μl cos θ I 0 cos 3 θ t d x, y cos θ = d d 2 +x 2 +y 2 at z = d Considering the magnification at arbitrary z, where 0 < z d; I d x, y = I 0 cos 3 θ t z or I d x, y = I 0 d x M(z), y M(z) d 2 + x 2 + y 2 3 t z xz d, yz d Reasonable approximation only for relatively thin objects that have nearly the same M(z) & no variation in attenuation in the z direction 34 17

18 Blurring effects Due to extended sources & the intensifying screen Extended sources (= finite focal spot size) Fuzziness both at the edge of the FOV & object boundaries Size of a point hole located at range z for the source w/ diameter D; D d z = D = m z D z m z = d z D z» source magnification» Negative because it inverts the image of the source = 1 M(z) 35 Source image for the source intensity distribution s(x, y) I d x, y = ks x m, y m m = the source magnification for a point at range z k = the amplitude scaling term» ks x m(z), y m(z) dxdy = constant» Fourier transform km 2 z S 0,0 = constant» k 1 m 2 z The amplitude of blurring due to the extended source depends on the location of the object relative to the detector plane s x m, y m S(0,0)δ(x, y) as m(z) 0 m 2 No loss of resolution or amplitude Spatial distribution of attenuating objects w/i a given z plane: I d x, y = cos 3 θ s x m, y m t 4πd 2 m 2 z x M, y M 36 18

19 37 Film-screen blurring Consider an additional convolution w/ the film-screen impulse response function h(x, y) assuming that light photons isotropically scatter: I d x, y = cos 3 θ s x m, y m t 4πd 2 m 2 z x M, y M h(x, y) Tradeoff btwn image resolution (or blur) & detector efficiency = the fraction of photons captured by the detector on average ~0.3 for CaWO

20 Film characteristics Usually negligible MTF of the film compared w/ the intensifying screen Ignoring the image created directly on the film by x rays due to very poor Only considering the image created by the light produced in the phosphors adjacent the film X rays captured by the screen converted to light photons & captured by the film a latent image (i.e., not visible) a blackening of the film Characterizing film as a transformation btwn exposure to light & the degree of blackening of film optical density of the film 39 Optical transmittivity = the fraction of light transmitted thru the exposed film T = I t I i I i = the irradiation of the incident light [energy/area/sec] I t = the irradiation of the transmitted light [energy/area/sec] Optical opacity = T 1 Optical density = the common logarithm of the optical opacity D = log 10 I i I t Characterizing how black the film in on a logarithmic scale Usable when 0.25 D 2.25 Best discrimination of shading gray when 1 < D <

21 H&D (Hurter & Driffield) curve S-shaped curve: Low D low exposure / linear portion / high D high exposure Base fog = non-zero optical density even in the absence of exposure In the linear region: D = Γ log 10 X X 0 X 0 = the exposure when the linear region would hit the horizontal axis (D = 0) Γ = the slope of the H&D curve in the linear region ( contrast )» Called the film gamma, typ » Γ contrast but lattitude Lattitude = the range of exposures over which the H&D curve is linear ( dynamic range ) Speed of film = the inverse of the exposure at which D = 1 + fog level

22 Noise & Scattering Signal-to-noise ratio (SNR) Consider the detector intensities from a rectangular object assuming unity magnification & infinitesimal source size: Local contrast C = I t I b I b Addition of noise due to the random fluctuation in # of photons arriving in each small area of detector Called quantum mottle Responsible for the impression of detector measurements of x-ray intensity 43 SNR = I t I b = C I b σ b σ b X-ray intensity in the number of photons using the effective energy for polyenergetic beam: I = Nhν AΔt Average background intensity: I b = N bhν AΔt Variance of the number of photons per burst per area A in the background: σ 2 hν 2 b = N b AΔt Then, the local SNR: SNR = C N b = C ΦARtη Φ = the number of photons per Roentgen per cm2 A = the unit area R = the body s radiation exposure in Roentgens t = the fraction of photons transmitted thru the body η = the detector efficiency 44 22

23 Quantum efficiency & detective quantum efficiency Quantum efficiency Probability that a sing photon incident upon the detector will be detected Detective quantum efficiency Considering the transformation of SNR from a detector s input to its output A measure of the degradation in the SNR due to the detection process The fraction of photons that are detected correctly DQE QE 1 DQE = SNR 2 out SNR in» SNR in = the intrinsic SNR of the incident radiation» SNR out = the SNR of the measured quantity 45 Compton scattering Resulting in a decrease in image contrast & a decrease in SNR Effect on image contrast Recall the local contrast C = I t I b I b Assuming that CS adds a constant intensity I s to both target & background intensities C = I t + I s I b + I s I b C = C = I b + I s I b + I s 1 + I s I b Scatter reduces contrast by the factor 1 (1 + I s I b ) I s I b called scatter-to-primary ratio SNR w/ scatter SNR = I t I b = C I b = C σ b σ b SNR = SNR 1+ I s I b N b N b +N s = C N b 1+ N s N b 46 23