Things they never tell you when the equipment is installed REV 11/2008 Digital Radiography Or Things they never tell you when the equipment is installed REV 11/2008

Computed Radiography Fundamentals of Computerized Radiography

Imaging Plates Latitude and Contrast Independent functions Contrast is dependent on look-up table (LUT) and final image processing Image appearance not as closely related to exposure factor selection as F/S was Except when extremely out of range

IMAGE READING

EDR Exposure Data Recognition When laser scans it is looking for area of plate that has exposure Some read from center out and look for two sides of collimation Works best when image centered

Histogram Identifies all densities on plate A graph that displays signal value x-axis related to amount of exposure y-axis displays number of pixels for each exposure Series of peaks and valleys Pattern varies for each body part

Histogram Location Example of a chest histogram X-ray energy and absorption effects Low kVp, large differences (high contrast) High kVp, small differences (low contrast) Different energies superimposed Different doses (mAs), same kVp

Darker Lighter Histogram showing pixel values in an image. The pixel values in gray are on the horizontal with the total number for each on the vertical.

Histogram Analysis Collimation is very important If plate reader cannot find collimated edges then all the exposure on plate will be included in the histogram Histogram from plate is compared to body part histogram stored in computer

Characteristic curve & histogram

Underexposed

Overexposed

Just right!

LUT Look Up Table (LUT) Each anatomic area has a LUT Used to adjust contrast and density Other terms that may be used for this Contrast rescaling Contrast processing Gradation processing Tone scaling

LUT The image data from the histogram is rescaled for application of the LUT The LUT maps the adjusted data through a “S” curve that is similar to an H & D curve The result is an image that has the correct contrast and brightness (density)

1. is unprocessed, 2. algorithm finds anatomy, 3. finished

LOOK UP TABLE (LUT) Black Saturation White Saturation Linear LUT Black Shirt One more concept in the image processor that is critical. It’s called the LUT. It stands for lookup table. The situation here is that if we take this picture and we say “Let’s take a look at how much of the image is black and how much is white and how much is gray”. This is a little fancy curve, it’s called a histogram. What you see is all the blacks are down here. This is a lot of her shirt down here. And this is the gray area. In fact, these are most of the facial tones. If you want to reproduce this image exactly as it was taken on the image intensifier, you want the whole thing to be linear. Every time you put this energy in, you want the exact out. Again, like anything else, I can play with this lookup table and say “Forget about the blacks. Everything that’s below this curve let’s make black. Because what I really want to do is I want to see the shades of gray in this white area”. Or I can do the reverse. I can say “Forget about all the white stuff. I want to see the shades of gray in here”. Now, why would anybody in their right mind want to do this? Easy. Radiography is a game of shades of gray. That’s all the radiologist is doing. He wants to see a little bit of a calcification here, a little bit of air in the lung here. It’s always a game of the shades of gray. So, the more you can take his area of interest and stretch it out for him, he’s going to be a happy camper. And he’s willing to compromise that the other areas are going to be bad. Look at this original image here. You’d say “That’s the image I’d want”. Can you tell me what kind of sweater she’s got on. Look at the ribbing. So, we might say, we’re not interested in her face and her hair, but we’re interested in the shades of gray in the vest, or in the cardiac or mediastinum in the medical world. So, the point here is that by varying lookup tables, we can actually also vary the ability to see varying shades of gray. This image processor can play games with low pass, high pass, or lookup tables to create images that are just spectacular, or garbage if you happen to set it up wrong. Facial Tones * No Detail in Black Areas * High Contrast * Only Detail in White Areas can be seen * No Detail in White Areas * Low Contrast * Only Detail in Black Areas can be seen

Exposure Indicators Imaging plates get a signal from the exposure they receive The value of the signal is calculated from the region identified as the anatomy of interest The signal for the plate is an average of all signals given to the plate

The total signal is not a measure of the dose to the patient but indicates how much radiation was absorbed by the plate A 1 mR exposure will give Fuji S# 200 Kodak EI 2,000 Agfa 200 speed lgm reference value for site

EXPOSURE VALUES Exposure indicator S number inverse to exposure Plates sensitive to 0.1 mR – 100 mR “S” number for Fuji S number inverse to exposure S=2 (100 mR), S=200 (1 mR) Kodak uses exposure index 2000 (1 mR), 3000 (10 mR)

Exposure Values Agfa has ADC dose monitoring Stored reference dose for each exam Based on range Compares image obtained to reference Indicates significant deviation Notes lgM, not patient dose, plate dose Doubling dose increases lgM by 0.3 (log)

Exposure Values Agfa lgM, log gradient mean Speed class, 200 or 400 Double dose is 0.3 since it is log Have stored reference dose for exams Set for hospital, each image compared to it Speed class, 200 or 400 Tells relative amount of light to be read from plate Allows electronics to be optimized to this range for better image data set

Agfa Agfa uses a speed system Here is a hand taken at 50, 200 and 400 speeds The mAs was changed for each one Lgm is the same for all

Using Exposure Numbers Fuji, if appropriate # is 200 then At 400, too light, double mAs for 200 At 100, too dark, half mAs for 200 Agfa, if appropriate # is Lgm 2.2 At 1.9, too light, double mAs for 2.2 At 2.5, to dark, half mAs for 2.2 Kodak, if appropriate # is 1800 At 1500, too light, double mAs for 1800 At 2100, to dark, half mAs for 1800

S# 47 S# 86 S# 16

S# 8,357 Machine shut down and closed collimator when turned back on. S# 12,361 lat CXR S# 8,357 S# 12,361 lat CXR

Exposure Numbers The exposure numbers can only be used if all other parameters are correct Centering to plate Collimation Position over AEC, look at mAs readout to determine if poor positioning caused light or dark image

Same technique, different centering and collimation

Exposure Values Each system has range of values for appropriate exposure for part The range used by vendor is very broad Each facility should develop its own exposure range taking into account Radiologist preference ALARA

Comparison of DR & CR

IMAGE CAPTURE CR DR – NO CASSETTE – PHOTONS PSP – photostimulable phosphor plate REPLACES FILM IN THE CASSETTE DR – NO CASSETTE – PHOTONS CAPTURED DIRECTLY ONTO A TRANSISTOR SENT DIRECTLY TO A MONITOR

DR TFT, thin film transistor Used under silicon or selenium Collects charge then sends out as signal to computer Releases e- line by line

TFT

CR – PSP plate Stimulated by a RED LIGHT Energy is RELEASED in a form of BLUE light LIGHT captured by PMT – changed to a digiial signal

How CR works Released light is captured by a PMT (photo multiplier tube) This light is sent as a digital signal to the computer The intensity (brightness) of the light – correlates to the density on the image

Densities of the IMAGE The light is proportional to amount of light received digital values are then equivalent (not exactly the same) to a value of optical density (OD) from a film, at that location of the image

ADVANTAGE OF CR/DR AFTER THE EXPOSURE CHANGES MADE TO IMAGE AFTER THE EXPOSURE CAN ELIMINATE THE NEED TO REPEAT THE EXPOSURE

ADVANTAGE OF CR/DR vs FS Rapid storage retrieval of images NO LOST FILMS! PAC (storage management) Teleradiology - long distance transmission of image information Economic advantage - at least in the long run?

CR/DR VS FILM/SCREEN FILM these can not be modified once processed If copied – lose quality DR/CR – print from file – no loss of quality

“no fault” TECHNIQUES F/S: RT must choose technical factors (mAs & kvp) to optimally visualize anatomic detail CR: the selection of processing algorithms and anatomical regions controls how the acquired latent image is presented for display HOW THE IMAGE LOOKS CAN BE ALTERED BY THE COMPUTER – EVEN WHEN “BAD” TECHNIQUES ARE SET

DR Initial expense high very low dose to pt – image quality of 100s using a 400s technique Therfore ¼ the dose needed to make the image

Histogram Analysis A histogram is a plot of gray scale value vs. the frequency of occurrence (# pixels) of the gray value in the image

HISTOGRAM – a bar graph depicting the density distribution (in numerical values) of the imaging plate ALGORITHM – a set of mathematical values used to solve a problem or find an average

Adapted from AAPM TG10

The algorithm attempts to distinguish among the parts of the histogram which represent the range of densities from bone to soft tissue

Histograms set for specific exams (body parts) should produce digital images that are consistant (regardless of kVp or mAs used Correct Algorithm (body part) must be selected prior to processing imaging plate

Methods to Digitize an Image 1. Film Digitizer - Teleradiography system (PACS, DICOM) 2. Video Camera (vidicon or plumbicon) 3. Computed Radiography 4. Direct Radiography

FILM DIGITIZER

ANALOG TO DIGITAL IMAGE Conversion of conventional analog films to digital format for PACs and teleradiology applications with scanning laser digitizers

CONTRAST & DENSITY Most digital systems are capable of 1024 shades of gray - but the human eye can see only about 30 shades of gray The Optical Density and Contrast can be adjusted after the exposure by the Radiographer. This is POST - PROCESSING

High displayed contrast – narrow window width

Low displayed contrast (stretched) – wide window width

Basics of Digital Images Pixel values can be any bit depth (values from 0 to 1023) Image contrast can be manipulated to stretched or contracted to alter the displayed contrast. Typically use “window width” and “window level” to alter displayed contrast

standard image edge sharpening

REPROCESSED HAND ALGO NO GRID

POSITIONING & PROPER COLLIMATION ARE CRITICAL TO GOOD IMAGING OUTCOMES Just like Phototiming, it can magnify your mistakes

COLLIMATION CRITICAL AS THE COMPUTER READS THE DENSITY VALUE OF EACH PIXEL – IT IS AVERAGED INTO THE TOTAL CLOSE COLLIMATION = BETTER CONTRAST BAD COLLIMATION = MORE GRAYS AND LESS DETAIL

A B

Digital imaging is not the end all, cure all for imaging problems. It is still technologist dependent. You must continue to think and apply everything learned in Imaging 101. A computer is not an intelligent machine. It can only perform as good as the information it was given.