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Screen Film Radiology Mohammad Reza AY, PhD Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran Division of Nuclear Medicine, Geneva University Hospital, Geneva, Switzerland Objectives of Lecture v Understand the principles of basic geometric principles and apply to projection imaging v How screen-film detector systems work v Define the characteristics of screens and films v Understand the relation between contrast and dose in radiography v Significance of scattered radiation in projection radiography 2 1 1. Projection Radiography v Projection imaging is the acquisition of a 2D image of a patient’s 3D anatomy v Projection radiography is a transmission imaging procedure v The optical density at any location on the film corresponds to the attenuation characteristics (eµx) of the patient at that location c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 146. 3 2. Basic Geometric Principles v Similar v v triangles (geometry) a/A = b/B = c/C = h/H d/D = e/E = f/F = g/G v Similar triangles are encountered when determining the image magnification and when evaluating image unsharpness caused by focal spot size and patient motion c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 147. 4 2 2. Basic Geometric Principles v v a/A = b/B = c/C = h/H d/D = e/E = f/F = g/G v Magnification v v v (M) Occurs because x-ray beam diverges from focal spot to image plane For a point source, v M = I/O = SID/SOD largest when object closest to focal spot and approaches value of 1 when object at image plane c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 147. 5 2. Basic Geometric Principles v For a extended source (focal spot), v Penumbra or blur (f) v edge gradient blurring due to finite size of focal spot (F) v f/F = OID/SOD v f/F = (SID-SOD)/SOD v f/F = (SID/SOD)-1 v f = F(M-1) v v f or blur increases with F and M f can be decreased by keeping object close to image plane (↓OID) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 147. 6 3 3. The Screen-Film Cassette Cassette v v v Light-tight and ensures screen contact with film Front surface - carbon fiber ID flash card area on back 1 or 2 Intensifying Screens v v Convert x-rays to visible light Mounted on layers of compressed foam (produces force) Sheet of film v v v Register the x-ray distribution Chemically processed Storage and display c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 148. 7 4. Characteristics of Intensifying Screens v Film relatively insensitive to x-rays, requires a lot of x-ray energy to produced a properly exposed x-ray film v Patient receives a large dose v To reduce dose and exposure times, screens are used made of scintillating material: phosphor v Screens v When x-rays interact with phosphor, visible or UV light is emitted Light emitted darkens the film v → Screen-film detectors are considered an Indirect detector Using filmfilm-screen versus film only reduces radiation dose to patient by a factor of 50! 8 4 4. Screen Composition and Construction 20th century: calcium tungstate, CaWO4 v Early v Since early 70’s: rare earth phosphor v Lanthanide series: Z = 57 – 71 v Gd2O2S:Tb (gadolinium oxysulfide: terbium) - common v LaOBr:Tm (lanthanum oxybromide: thulium) v YTaO4:Nb (yttrium tantalate: niobium) c.f. http://www.ktf- split.hr/periodni/en/ 9 4. Screen Composition and Construction v Top coat v Phosphor and binder v Adhesive v Support thickness (g/cm2) expressed as mass thickness = thickness (cm) · density (g/cm3) v Phosphor v v General radiography: each of two screens around 60 mg/cm2 v Mammography: single screen of 35 mg/cm2 used c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 150. Cross-sectional image of an intensifying screen 10 5 4. Intensifying Screen Function and Geometry v Function: absorb x-rays, convert to visible or UV light which exposes the film emulsion v Conversion efficiency of a phosphor = fraction of absorbed energy emitted as UV or visible light v CaWO4 ≈ 5% intrinsic conversion efficiency v Gd2O2S:Tb ≈ 15% intrinsic conversion efficiency 50,000 eV x-ray x 0.15 = 7500 eV Green light, 2.7 eV 7500 eV / 2.7 eV/photon = 2,800 photons 200-1000 photons reach film after diffusing through phosphor layer and being reflected at the interface layers 11 4. Intensifying Screen Function and Geometry v Quantum Detective Efficiency (QDE) of a screen = fraction of incident x-rays photons that interact with it v QDE increases with screen thickness c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 151. 12 6 4. Intensifying Screen Function and Geometry v Thicker screens absorb greater amount of x-rays, but a greater lateral spread of the visible light occurs (isotropic diffusion) causing blurring and reducing spatial resolution Cross-section of a screen-film cassette vA thin screen results in less x-ray absorption but less lateral spread of light and better spatial resolution v For maximum resolution, a singlescreen cassette is used v v X-rays first traverse the film and then strike screen Less light spread and maximum spatial resolution ↑ Radiography ↑ Mammography c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 152. 13 4. Intensifying Screen Function and Geometry v As screen thickness ↑ QDE ↑ and screen sensitivity ↑, but light-diffusion increases v Compromise between sensitivity and resolution c.f. http://www.sprawls.org/resources/RADDETAIL/classroom.htm c.f. http://www.rad-icon.com/pdf/Radicon_AN07.pdf 14 7 4. Intensifying Screen Function and Geometry v Modulation Transfer Function (MTF) describes the resolution properties of an imaging system v The MTF illustrates the fraction (or %) of an object’s contrast that is recorded by the imaging system as a function of object size (spatial frequency) vF (linepairs or cycles/mm) F=1/2∆, ∆ = object size v As screen thickness ↑ MTF ↓ c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 152. 15 4. Intensifying Screen Function and Geometry v Crossover or print-through: light from top screen penetrates the film base and exposes the bottom emulsion or vice versa v Due to type of film grain, not a problem now c.f. http://www.sprawls.org/resources/RADDETAIL/classroom.htm 16 8 4. Conversion Efficiency (CE) Total conversion efficiency (CE) of a screen-film combination refers to the ability of the screen or screens to convert the energy deposited by the absorbed x-rays into film darkening or optical density CE depends on: v Intrinsic conversion efficiency of phosphor v Efficiency of light propagation through the screen to film emulsion layer v Efficiency of the film emulsion in absorbing the emitted light c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 153. 17 4. Conversion Efficiency (CE) Light propagation in screen (diffuses in all directions) Distance from absorption to film Light-absorbing dye reduces lateral distance: CE ↓ (slow), Spatial resolution or MTF ↑ Reflective layer redirect light photons: CE ↑ (fast), Spatial resolution or MTF ↓ Screen is a linear device at a given x-ray energy v Number of x-ray photons doubles, light intensity produced by screen also doubles c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p 153. 18 9 4. Absorption Efficiency (AE) v The absorption efficiency or QDE describes how efficiently the screen detects x-ray photons that are incident upon it v X-ray photon absorbed by the screen deposits its energy and some fraction of energy is converted to light photons v Screen-film systems are energy detectors The number of light photons produced in the screen is determined by the total amount of x-ray energy absorbed by the screen, not by the number of x-ray photons c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 154-155. 19 4. Overall Efficiency of a Screen-Film System v Total efficiency = AE · CE vA SF system increases x-ray detection efficiency compared to film only (29.5% vs. 0.65% at 80 kVp) v Using film-screen versus film only reduces radiation dose to patient by a factor of 50! v Intensification factor (IF) = ratio of energy absorption of 120 mg/cm2 phosphor vs. 0.80 mg/cm2 AgBr v An IF of 50 is achieved for Gd2O2S over the x-ray energy spectra commonly used for diagnostic examination c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 156. 20 10 4. Noise Effects of Changing CE vs. AE Noise: local variations in film OD, not representing variations of attenuation in patient Includes random noise caused by factors such as v Statistical fluctuation in x-ray quantity interacting with screens v v Statistical fluctuation in fraction of light emitted by the screen that is absorbed by the film emulsion Statistical fluctuation in the distribution of silver halide grains in film emulsion The visual perception of noise is reduced (better image quality) when the number of detected x-ray photons increases (more in Chapter 10) 21 Noise Effects on Image Quality 22 c.f. http://www.sprawls.org/resources/IMGCHAR/module/#20 11 4. Noise Effects of Changing CE vs. AE What happens to noise in image when the CE is increased (or fast film screen system) by adding reflective layer? If “speed” of the SF system is increased by increasing the CE (so that each detected x-ray photon becomes more efficient at darkening the film): v few x-ray photons are required to achieve same film darkening (as before increasing CE), so noise increases v Therefore, increasing the CE to increase the speed of a SF system will increase the noise in the images 23 4. Noise Effects of Changing CE vs. AE What happens to noise in image when the AE is increased (thicker screen)? If AE is increased, 10% more x-ray photons detected, then reduction of 10% in incident x-ray beam is required to deliver same amount of film darkening (as before increasing AE) v v Since the fraction of increase in x-ray photon detection and reduction in incident x-ray intensity is same, the total number of detected x-ray photons is the same. No change in noise However, spatial resolution will get worse with thicker screens 24 12 5. Characteristics of Film v1 or 2 layers of film emulsion coated onto a flexible Mylar plastic sheet v Emulsion: silver halide (AgBr and AgI) bound in a gelatin base Tubular grains Cubic grains v Emulsion of an exposed sheet of film contains the latent image v Latent image rendered visible through film processing by chemical reduction of silver halide into metallic silver grains c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 157. 25 Optical Density v Increased x-ray exposure → developed film becomes darker v Degree of darkness of the film is quantified by the optical density (OD) which is measured with a densitometer v Transmittance (T) is the fraction of incident light passing through the film v T = I/I0 where I – intensity measured at a particular location on film and I0 – intensity of light measured with no film in densitometer c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 158. 26 13 Optical Density v OD = -log10(T) -OD = log10(1/T) = log10(I0/I), inverse relationship is T = 10 v As OD increases, transmittance decreases v The OD of superimposed films is additive c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 158. 27 The Hurter and Driffield (H&D) Curve v H&D (characteristic) curve describes how film responds to x-ray exposure v Non-linear, sigmoidal shape v log10-log10 plot (OD vs. log relative exposure) v Film base → OD = 0.11 – 0.15 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 159. 28 14 The Hurter and Driffield (H&D) Curve v Fogging due to long storage, heat and low background exposure v Base + Fog ≤ 0.20 OD v Toe v Linear region v Shoulder c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 159. 29 Contrast of Film (Average Gradient) v Contrast of film is related to the slope of the H&D curve: v Higher slope have higher contrast v Reduced slope have lower contrast v Overall contrast given by Average gradient = v [OD2-OD1]/[log10(E2)log10(E1)] v OD2 = 2.0 + B + F v OD1 = 0.25 + B + F v Range from 2.5 – 3.5 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 160. 30 15 Contrast of Film (Average Gradient) v Describes the contrast properties of the film-screen system v Important to obtain well controlled exposure levels to ensure good contrast v Film manufacturer physically controls contrast on film by varying the size distribution of the sliver grains 31 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 161. Sensitivity or Speed v As the speed of SF system increases, the amount of x-ray exposure required to achieve same OD decreases v Fast films requires less exposure to achieve a given OD; slow films require more exposure v Faster (higher-speed) SF systems result in lower patient doses but in general exhibit more quantum mottle (noise) than slower systems c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 162. 32 16 Sensitivity or Speed v Absolute speed = 1 / Exposure (R) required to achieve OD = 1.0 + B+F v Relative speed of a SF combination– relative to a common standard (100 speed), commercially used v Most US institutions that use screen-film use 400 speed for general radiography c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 162. 33 Sensitivity or Speed v 100-speed – detail work bony radiographs of extremities, (thinner screens, slower, better spatial resolution) v 600-speed – angiography (thicker screens, decreased spatial resolution) c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 162. 34 17 Latitude (Dynamic Range) v Horizontal shift between 2 H&D curves – systems differ in speed v Systems with different contrast have H&D curves with different slopes v Latitude is the range of xray exposures that deliver ODs in the usable range v Latitude is also called dynamic range c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 162. 35 Latitude (Dynamic Range) v System A has higher contrast but reduced latitude v It is more difficult to consistently achieve proper exposures with low-latitude SF systems. v Chest radiography needs a high-latitude system to achieve adequate contrast in both the mediastinum and lung fields c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 162. 36 18 6. The Screen-Film System v Film emulsion should be sensitive to light emitted by screen v CaWO4 emits blue light to which film is sensitive v Gd2O2S:Tb emits green light v Wavelength sensitizers added to film v Screens and films usually purchased in combination since matching of spectral sensitivity very important 37 Reciprocity Law of Film v Reciprocity law of film states that the relationship between exposure and OD should remain constant regardless of the exposure rate v Reciprocity law failure: at long and short exposure times, the film becomes less efficient at using the light incident on it and lower ODs result v This is a factor in mammography when long exposure times are needed for large and dense breasts All points exposed exactly the same with different exposure rate 38 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2st ed., p. 163. 19 7. Contrast and Dose in Radiography v The SF system governs the overall detector contrast v The contrast of a specific radiographic study depends on the requirements of the study, total exposure time, radiation dose, size of patient and so on… v The kVp (quality) and mAs (quantity) are adjusted by the technologist to adjust the subject contrast v v Quality – energy or penetrating power of x-ray beam (As kVp increases, HVL also increases) Quantity – number of x-ray photons Technique still an art, but: v v Technique chart Phototimer (automatic technique) 39 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 165. Contrast and Dose in Radiography (2) v kVp ↑ → dose and contrast ↓ v Classic compromise between image contrast and patient dose c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 165-166. 40 20 8. Scattered Radiation in Projection Radiography v Most radiographic interactions produce scattered photons v Scattered photons → violation of the basic principle of projection imaging: mis-information reducing contrast v The scattered photon if detected by film causes film darkening but provides no useful information to the image v Scatter-to-primary (S/P) ratio refers to how many scattered xray photons there are for every primary photon c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 167. 41 Scattered Radiation in Projection Radiography Scatter-to-Primary ratio (S/P) v Area of collimated x-ray field v Object thickness v kVp of x-ray beam v As FOV is reduced, scatter is reduced c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 167. 42 21 Scattered Radiation in Projection Radiography v Scatter radiation causes loss of contrast v In the absence of scatter, for two adjacent areas transmitting photon fluences of A and B, the contrast is: v C0 = [A-B]/A v In the presence of scatter: v C = C0 x [1 / (1 + S/P)] v S/P ↑ → contrast ↓ v 1/(1+S/P): contrast reduction factor c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 168. 43 The Antiscatter Grid v The antiscatter grid is used to combat the effects of scatter v Between object and detector v Uses geometry to ↓ scatter v Thin lead septa separated by aluminum or carbon fiber, aligned with focal spot v Grid ratio (GR) = H/W = septa height/interspace width v 8:1, 10:1 and 12:1 common, 5:1 for mammography v↑ v↑ GR → ↓ S/P GR → ↑ dose c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 168-169. 44 22 The Antiscatter Grid GR → ↑ clean-up of scatter striking the grid at large angles, less effective for smaller angles v↑ v v Grid frequency: lines/cm grid freq. doesn’t alter S/P 60 lines/cm c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 170. 45 The Antiscatter Grid v Stationary grids: lines appear on image v Bucky: device that moves grid v Moving grid bars on visible on image v Bucky v factor = dosew grid/dosew/o grid v Bucky v factors: Range from 3 to 5 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 171. 46 23 Grid Artifacts v Most grid artifacts due to mispositioning v Upside down: severe loss of OD at margins v Crooked & off-center: general decrease of OD across entire image v Off-focus: edges loss at lateral c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 172. 47 Air Gaps v Air gap: ↓ S/P, but ↑ M, ↓ FOV and ↓ MTF (unless very small focal spot used) v Not used all that often in radiography except chest radiography, used in mammography c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 173. 48 24 IMAGE QUALITY INFLUENCE OF ENERGY (kV) Bone Soft tissue THE CORRECT ENERGY Good difference between dense and soft tissues Best contrast TOO HIGH ENERGY Very few photons are absorbed Small difference between dense and soft tissues Low contrast TOO LOW ENERGY Most of the photons are absorbed Low signal Dose to the patient with no usable output • High energy may be necessary in dense or thick areas (abdomen, pelvis) • Very low energies are rarely of use and are harmful to the patient 25 INFLUENCE OF ENERGY (kV) kV Loss of contrast Pale image Loss of contrast Dark image High High contrast Dark image Lack of photons going through Pale and loss of information Low Low High mAs INFLUENCE OF ENERGY (kV) Low kV High kV Patient dose Penetration Contrast The choice of kV depends on the patient anatomy and the object to be imaged • Thick or dense part of the body: High kV, ex. Abdomen, pelvis - 80 to 100 kV • Small or flat parts of the body: Low kV, ex. Breast, hand - 30 to 50 kV Then the necessary mAs is elected to produce the clinically useful image 26 INFLUENCE OF FOCAL SPOT SIZE: PENUMBRA Fine (small) focal spot Sharp projection from a fine focal spot Dark Dark Clear Large focal spot Large focal spot is responsible for blurr, leading to inability to distinguish between small, close objects Dark Dark Clear Gray Gray LOSS OF SPATIAL RESOLUTION Penumbra INFLUENCE OF FOCAL SPOT SIZE: PENUMBRA Increased SOD Decreased OID SOD SID OID The blurr effect, or Penumbra, can be decreased by: • Increasing the Source to Object Distance • Placing the image receptor as close as possible to the patient 27 MAGNIFICATION EFFECT long OID short OID There is always a magnification, but the same object may appear with different size depending on its location in the body The differential magnification is also reduced by using long SOD and image receptor close to the patient SIGNAL TO NOISE RATIO (SNR) IMPROVING SNR Less Noise The better the SNR The better the visibility LOSS OF CONTRAST RESOLUTION Signal Inherent object contrast Amplitude of the signal (difference in contrast versus the local background) SNR = Amplitude of the noise (random fluctuation of the signal) 28 INCREASING THE SIGNAL - CONTRAST AGENTS A way to improve SNR Early phase: opacification of the general arterial system Injection of an Iodine solution in the antecubital vein (arm) Latter phase: opacification of the urinary system (IVU) Local injection of Iodine through a catheter Local vascular opacification: here the coronary arteries Catheter Thanks For Your Attention 29