Screen Film Radiology

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 (e - 

µx ) of the patient at that 

location 

c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p 146. 

2. Basic Geometric Principles 

v Similar triangles (geometry) 

v a/A = b/B = c/C = h/H 

v 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 


3 

4 

2


v a/A = b/B = c/C = h/H 

v d/D = e/E = f/F = g/G 

v Magnification (M) 

v Occurs because x-ray beam 

diverges from focal spot to 

image plane 

v For a point source, 

v M = I/O = SID/SOD 

v largest when object closest 

to focal spot and 

approaches value of 1 when 

object at image plane 



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 f or blur increases with F and M 

v f can be decreased by keeping 

object close to image plane 

(↓OID) 

c.f. Bushberg, et al. The Essential Physics of 

Medical Imaging, 2 nd ed., p 147. 

5 

6 

3

3. The Screen-Film Cassette 

Cassette 

v Light-tight and ensures screen 

contact with film 

v Front surface - carbon fiber 

v ID flash card area on back 

1 or 2 Intensifying Screens 

v Convert x-rays to visible light 

v Mounted on layers of 

compressed foam (produces 

force) 

Sheet of film 

v Register the x-ray distribution 

v Chemically processed 

v Storage and display 



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 

v Screens made of scintillating material: phosphor 

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 film-screen film screen versus film only reduces 

radiation dose to patient by a factor of 50! 

7 

8 

4

4. Screen Composition and Construction 

v Early 20 th century: calcium 

tungstate, CaWO 4 

v Since early 70’s: rare earth 

phosphor 

v Lanthanide series: Z = 57 – 71 

v Gd 2 O 2 S:Tb (gadolinium 

oxysulfide: terbium) - common 

v LaOBr:Tm (lanthanum 

oxybromide: thulium) 

v YTaO 4 :Nb (yttrium tantalate: 

niobium) c.f. http://www.ktf- split.hr/periodni/en/ 

4. Screen Composition and Construction 

v Top coat 

v Phosphor and binder 

v Adhesive 

v Support 

v Phosphor thickness (g/cm 2 ) 

expressed as mass thickness = 

thickness (cm) · density (g/cm 3 ) 

v 

v General radiography: each of 

two screens around 60 mg/cm 2 

v Mammography: single screen of 

35 mg/cm 2 used 


Cross-sectional image of 

an intensifying screen 

9 

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 CaWO 4 ≈ 5% intrinsic conversion efficiency 

v Gd 2 O 2 S: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 


v Quantum Detective Efficiency 

(QDE) of a screen = fraction of 

incident x-rays photons that 

interact with it 

v QDE increases with screen 

thickness 



11 

12 

6


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 

v A 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 X-rays first traverse the film and 

then strike screen 

v Less light spread and maximum 

spatial resolution 

Cross-section of a screen-film cassette 

↑ 

↑ 

Radiography Mammography 




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 

13 

14 

7


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) 

v F (linepairs or cycles/mm) 

F=1/2∆, ∆ = object size 

v As screen thickness ↑ MTF ↓ 




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 

15 

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 



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 


17 

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 


Medical Imaging, 2 nd ed., pp. 154-155. 

4. Overall Efficiency of a Screen-Film System 

v Total efficiency = AE · CE 

v A 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/cm 2 

phosphor vs. 0.80 mg/cm 2 AgBr 

v An IF of 50 is achieved for Gd2O2S over the x-ray 

energy spectra commonly used for diagnostic 

examination 


Medical Imaging, 2 nd ed., p. 156. 

19 

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 Statistical fluctuation in fraction of light emitted by the 

screen that is absorbed by the film emulsion 

v 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) 

Noise Effects on Image Quality 

c.f. http://www.sprawls.org/resources/IMGCHAR/module/#20 

21 

22 

11


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 


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 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 

v However, spatial resolution will get worse with thicker 

screens 

23 

24 

12

5. Characteristics of Film 

v 1 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 

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 

Optical Density 

Tubular 

grains 



v Increased x-ray exposure → developed film 

becomes darker 

Cubic 

grains 

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/I 0 where I – intensity measured at a particular 

location on film and I 0 – intensity of light measured 

with no film in densitometer 


Medical Imaging, 2nd ed., p. 158. 

25 

26 

13

Optical Density 

v OD = -log 10 (T) = log 10 (1/T) = log 10 (I 0 /I), inverse relationship is T 

= 10 -OD 

v As OD increases, transmittance decreases 

v The OD of superimposed films is additive 


Medical Imaging, 2nd ed., p. 158. 

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 log 10 -log 10 plot (OD vs. log 

relative exposure) 

v Film base → OD = 0.11 – 0.15 



27 

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 



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 [OD 2 -OD 1 ]/[log 10 (E 2 )log 

10 (E 1 )] 

v OD 2 = 2.0 + B + F 

v OD 1 = 0.25 + B + F 

v Range from 2.5 – 3.5 c.f. Bushberg, et al. The Essential Physics of 

Medical Imaging, 2 nd ed., pp. 160. 

30 

29 

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 

c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd 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 



31 

32 

16


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 


v 100-speed – detail work 

bony radiographs of 

extremities, (thinner screens, 

slower, better spatial 

resolution) 

v 600-speed – angiography 

(thicker screens, decreased 

spatial resolution) 





33 

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 


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 



35 

36 

18

6. The Screen-Film System 

v Film emulsion should be sensitive to light emitted 

by screen 

v CaWO 4 emits blue light to which film is sensitive 

v Gd 2 O 2 S: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 

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 

c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 st ed., p. 163. 

All points exposed exactly the same with different 

exposure rate 

37 

38 

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 Quality – energy or penetrating power of x-ray beam (As kVp 

increases, HVL also increases) 

v Quantity – number of x-ray photons 

Technique still an art, but: 

v Technique chart 

v Phototimer (automatic technique) 


Contrast and Dose in Radiography (2) 

v kVp ↑ → dose and contrast ↓ 

v Classic compromise between image contrast and patient 

dose 


Medical Imaging, 2 nd ed., pp. 165-166. 

39 

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 



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 



41 

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 C 0 = [A-B]/A 

v In the presence of scatter: 

v C = C 0 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 


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 ↑ GR → ↓ S/P 

v ↑ GR → ↑ dose 

c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., pp. 168-169. 

43 

44 

22


v ↑ GR → ↑ clean-up of 

scatter striking the grid at 

large angles, less effective 

for smaller angles 

v 

v Grid frequency: lines/cm 

grid freq. doesn’t alter S/P 

60 lines/cm 


v Stationary grids: lines 

appear on image 

v Bucky: device that moves 

grid 

v Moving grid bars on visible 

on image 

v Bucky factor = 

v dose w grid /dose w/o grid 

v Bucky factors: 

v Range from 3 to 5 





45 

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: loss at lateral 

edges 


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 



47 

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


kV 

High 

Low 

Low 

Loss of contrast 

Pale image 

Lack of photons 

going through 

Pale and loss of 

information 

High 


Patient dose 

Penetration 

Contrast 

Loss of contrast 

Dark image 

High contrast 

Dark image 

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 

Low kV High kV 

Then the necessary mAs is elected to produce the clinically useful image 

mAs 

26

INFLUENCE OF FOCAL SPOT SIZE: PENUMBRA 

Dark Clear Dark 

Dark 

Gray 

Clear 

Penumbra 

Fine (small) focal spot 

Large focal spot 

Gray 

Dark 

Sharp projection from a fine focal 

spot 

Large focal spot is 

responsible for blurr, 

leading to inability to 

distinguish between 

small, close objects 

LOSS OF SPATIAL RESOLUTION 

INFLUENCE OF FOCAL SPOT SIZE: PENUMBRA 

SOD 

OID 

SID 

Increased 

SOD 

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 

Decreased 

OID 

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) 

Less 

Noise 

SNR = 

IMPROVING SNR 

Signal 

Inherent object contrast 

Amplitude of the signal (difference in contrast versus the local background) 

Amplitude of the noise (random fluctuation of the signal) 

The better the SNR 

The better the visibility 

LOSS OF CONTRAST 

RESOLUTION 

28

INCREASING THE SIGNAL - CONTRAST AGENTS 

A way to improve SNR 

Injection of an Iodine 

solution in the antecubital 

vein (arm) 

Local injection 

of Iodine 

through a 

catheter 

Early phase: 

opacification of the 

general arterial 

system 

Latter phase: 

opacification of the 

urinary system 

(IVU) 

Local vascular 

opacification: 

here the 

coronary arteries 

Catheter 

Thanks For Your Attention 

29

Screen Film Radiology

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?