Fundamentals of Digital Radiology George David Medical College of Georgia

Filmless Department What we mean by Digital Digital Radiographs PACS Picture Archival & Communication Systems Reading from Monitors

What is a digital image? 2D array of #’s representing some image attribute such as optical density x-ray attenuation Radiography Fluoroscopy CTDI echo intensity Magnetization T1 T2 Proton Density 125 25 311 111 182 222 176 199 192 85 69 133 149 112 77 103 118 139 154 120 145 301 256 223 287 225 178 322 325 299 353 333 300

Number Array Forms Digital Image 194 73 22

Digital Image Formation The finer the mesh, the better the digital rendering

What is this? 12 X 9 Matrix

Same object, smaller squares 24 X 18 Matrix

Same object, smaller squares 48 X 36 Matrix

Same object, smaller squares 96 X 72 Matrix

Same object, smaller squares 192 X 144 Matrix

Display of Digital Image Each number of a digital image (pixel value) assigned a gray shade Assignments can be changed Window/level Pixel values cannot

Computer Storage Computer image file is array of numbers Same as any computer file 125, 25, 311, 111, 182, 222, 176, 199, 192, 85, 69, 133, 149, 112, 77, 103, 118, 139, 154, 125, 120, 145, 301, 256, 223, 287, 256, 225, 178, 322, 325, 299, 353, 333, 300 125 25 311 111 182 222 176 199 192 85 69 133 149 112 77 103 118 139 154 120 145 301 256 223 287 225 178 322 325 299 353 333 300

Digital Copies 125, 25, 311, 111, 182, 222, 176, 199, 192, 85, 69, 133, 149, 112, 77, 103, 118, 139, 154, 125, 120, 145, 301, 256, 223, 287, 256, 225, 178, 322, 325, 299, 353, 333, 300 125, 25, 311, 111, 182, 222, 176, 199, 192, 85, 69, 133, 149, 112, 77, 103, 118, 139, 154, 125, 120, 145, 301, 256, 223, 287, 256, 225, 178, 322, 325, 299, 353, 333, 300 = If you’ve got the same numbers you have an IDENTICAL copy Analog copies are never identical

All Digital Copies are Originals =

Image Matrix & Image Size Doubling the matrix dimension quadruples the # pixels 111 87 118 155 125 25 311 111 199 192 85 69 77 103 118 139 145 301 256 223 4 X 4 Matrix 16 pixels 2 X 2 Matrix 4 pixels

Doubling the matrix dimension quadruples # pixels Image Matrix Doubling the matrix dimension quadruples # pixels Matrix # Pixels 512 X 512 => 262,144 1024 X1024 => 1,048,576 2048 X2048 => 4,194,304 A 10242 matrix compared to a 5122 matrix quadruples disk storage requirements image transmission time digital image manipulation

Matrix Size & Resolution More pixels = better spatial resolution

Pixel Values & The Bit Bit=>Fundamental unit of computer storage Only 2 allowable values 1 Computers do all operations with 0’s & 1’s BUT Computers group bits together

Abbreviations Review Bit (binary digit) Byte Kilobyte Megabyte Smallest binary unit; has value 0 or 1 only Byte 8 bits Kilobyte 210 or 1024 bytes sometimes rounded to 1000 bytes Megabyte 213 or 1,048,576 bytes or 1024 kilobytes sometimes rounded to 1,000,000 bytes or 1,000 kilobytes

# of unique values which can be represented by 1 bit 2

# of unique values which can be represented by 2 bits 1 2 4 unique combinations / values 3 4

# of unique values which can be represented by 3 bits 5 1 6 2 7 3 8 4 8 unique combinations / values

Digital Image Bit Depth the number of computer bits (1’s or 0’s) available to store each pixel value Values Bits # Values 1 2 3 . 8 0, 1 00, 01, 10, 11 000, 001, 010, 011, 100, 101, 110, 111 . 00000000, 00000001, ... 11111111 2 1 = 2 2 2 = 4 2 3 = 8 . 2 8 = 256

Bit Depth and Pixel Presentation on Image Indicates # of possible brightness levels for a pixel presentation of brightness levels pixel values assigned brightness levels brightness levels can be manipulated without affecting image data window level

Bit Depth & Contrast Resolution The more bits per pixel the more possible gray shades and the better contrast resolution. 2 bit; 4 grade shades 8 bits; 256 grade shades

Computer Storage / Image Size Storage = # Pixels X # Bytes/Pixel Example: 512 X 512 pixels; 1 Byte / Pixel 512 X 512 pixel array # pixels = 512 X 512 = 262,144 pixels 262,144 pixels X 1 byte / pixel = 262,144 bytes 256 Kbytes 0.25 MBytes

Image Size Depends on both matrix size & bit depth Larger (finer) matrix requires more storage doubling matrix size quadruples image size Larger bit depth requires more storage doubling bit depth (theoretically) doubles image size

Image Compression jpg gif (20) 37’s reduction of digital image storage size by application of algorithm for example, repetitive data could be represented by data value and # repetitions rather than by repeating value jpg 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37 gif (20) 37’s

Image Compression Image Decompression Compression Ratio calculating original digital image from previously compressed data Compression Ratio original image size -------------------------------- compressed image size ratio depends upon data to be compressed algorithm

Compression Types Reversible Compression Non-reversable Compression Image decompresses to original pixel values Low compression ratios only Non-reversable Compression Decompressed image’s pixel values not necessarily identical to original much higher compression ratios possible variation from original image may or may not be visible or clinically significant

Non-Reversable Compression variation from original image generally increases with increasing compression ratio but a higher compression ratio means less storage requirements variation less noticeable for dynamic (moving) images than for still images such as radiographs

Computed Radiography (CR) Re-usable metal imaging plates replace film & cassette Uses conventional bucky & x-ray equipment                                                                  

CR Exposure & Readout

CR Readout

Another View: CR Operation

Computer Radiography (CR) photostimulable phosphor plate radiation causes electrons to move to higher energy states Excitation Plate’s structure traps electrons in higher energy states Form latent image H i g h e r E n e r g y - E l e c t r o n S t a t e P h o t o n p u m p s e l e c t r o n t o X - R a y h i g h e r e n e r g y s t a t e P h o t o n - - - L o w e r E n e r g y - - - - - - E l e c t r o n - - - - - - - - - S t a t e - - - - - - - - -

Reading Imaging Plate laser scans plate with laser releases electrons trapped in high energy states electrons fall to low energy states giving up energy as visible light light intensity is measure of incident radiation Lower Energy Electron State

Reading Imaging Plate Reader scans plate with laser Beam moved using rotating mirror Plate pulled through scanner by rollers Light emitted by plate measured by PM tube & recorded by computer

Laser & Emitted Light are Different Colors Phosphor stimulated by laser light Intensity of emitted light indicates amount of radiation incident on phosphor at each location Only light emitted by phosphor measured by PMT Filters remove laser light

CR Erasure after read-out, plate erased using a bright light plate can be erased and re-used Erasure re-use cycle can be repeated without limit Plate life defined not by erasure cycles but by physical wear

CR Resolution Some vendor CR spatial resolution depends upon plate size. Smaller pixels More pixels / mm

CR Throughput Generally slower than film processing CR reader must finish reading one plate before starting the next Film processors can run films back to back

CR Latitude Much greater latitude than screen/film Plate responds to many decades of input exposure under / overexposures unlikely Computer scale inputs exposure to viewable densities Unlike film, receptor separate from viewer

Film Screen vs. CR Latitude CR Latitude: .01 – 100 mR Unlike film, small changes in incident radiation result in CR signal 100

Digital Radiography (DR) Digital electronic bucky

DR Formats Electronic bucky incorporated into x-ray equipment Electronic wireless cassettes

Digital Radiography (DR) Receptor provides direct digital output No processor / reader required Images available virtually immediately Far fewer steps for radiographer

TFT = THIN-FILM TRANSISTOR ARRAY Types of DR Receptors TFT = THIN-FILM TRANSISTOR ARRAY

Digital Radiography (DR) High latitude as for CR DR portables now in available Radiographer immediately sees image

Digital Raw Image Unprocessed image as read from receptor CR Intensity data from PMT’s as a result of scanning plate with laser DR Raw Data read directly from TFT array Not a readable diagnostic image Requires computer post-processing Specific software algorithms must be applied to image prior to presenting it as finished radiograph

Enhancing Raw Image (Image Segmentation) * Identify collimated image border Separate raw radiation from anatomy Apply appropriate tone-scale to image Done with look-up table (LUT) This process is specific to a particular body part and projection

Image Segmentation Computer then defines anatomic region Computer establishes location of collimated border of image based on exam type and view provided by operator Computer then defines anatomic region Finished image produced by tone scaling Requires histogram analysis of anatomic region

Histogram Graph showing how much of image is exposed at various levels # pixels

Tone Scaling Post-Processing Body part & projection-specific algorithms determine average exposure Must correctly identify anatomical region Look Up Table (LUT) computed to display image with proper Density Contrast

Film/Screen Limited Latitude Film dictates proper radiation exposure No post processing Improperly exposed films lose contrast

Should I Worry? In CR & DR, image density is no longer a reliable indicator of exposure factor control.

CR / DR Latitude DANGER Will Robinson!!! Almost impossible to under or overexpose CR / DR Underexposures look noisy Overexposures look GOOD!!!

How to Determine Optimum Radiation Dose?

Carestream Exposure Index Each manufacturer provides feedback to technologist on exposure to digital receptor Displayed on CR reader monitor Displayed on workstations Fuji S-Number Carestream Exposure Index Deviation Index

Exposure Index Indexes differ by manufacturer Form Linear Logrithmic Some indexes go up with radiation, some go down

Calculated Exposure Index Affected by X-Ray technique selection Improper centering Improper selection of study or projection Placing two or more views on same CR cassette

Phototimed CR Phantom Image 75 kVp 88 mAs 2460 EI

Let’s Approximately Double mAs 75 kVp 88 mAs 2460 EI 75 kVp 160 mAs 2680 EI

Let’s Go Crazy 75 kVp 88 mAs 2460 EI 75 kVp 640 mAs 3300 EI

How Low Can You Go? Cut mAs in Half! 75 kVp 88 mAs 2460 EI 75 kVp 40 mAs 2060 EI

Let’s Go Crazy Low 75 kVp 8 mAs 1380 EI 75 kVp 1 mAs 550 EI

CR Artifacts Physical damage to imaging plates Dirt in reader Cracks, scuffs, scratches Contamination Dust / dirt Dirt in reader Highly sensitive to scatter radiation

DR Artifacts Dead detector elements Spatial variations in background signal & gain Grid interference Software can help correct for above

Shifting Gears: Fluoroscopy Issues

Digital Video Sources DR type image receptor Conventional Image Intensifier with Video Signal Digitized (“Frame Grabber”) I m a g e T u b X-Ray Input Image Tube TV Amplfier Analog to Digital Converter Memory (Computer) Lens System

Digital Spot Film Frame grabber digitizes image Digital image saved by computer Radiographic Technique used required to control quantum noise

Last Image Hold Computer displays last fluoro image before radiation shut off. Image noisier than for digital spot Image made at fluoroscopic technique / intensity Allows operator to review static processes without keeping beam on ideal for teaching environments ideal for orthopedic applications such as hip pinning Less radiation than digital spot

Fluoro Frame Averaging Conventional fluoro only displays current frame Frame averaging allows computer to average current with user-selectable number of previous frames Averages current frame & history

Fluoro Frame Averaging Tradeoff Advantage: Reduces quantum noise Disadvantage Because history frames are averaged with current frame, any motion can result in lag

Other Fluoro Features Real-time Edge Enhancement / Image Filtering Option of using lower frame rates (15, 7.5, 3.75 fps rather than 30) computer displays last frame until next one reduces flicker Lowers patient and scatter exposure Exposure proportional to frame rate dynamic studies may be jumpy

Digital Subtraction Immediate replay of run Free selection of mask before or after bolus >1 frame may be averaged for mask Note subtraction adds noise

Digital Image Manipulations on-screen measurements distances angles volumes/areas stenosis image annotation peak opacification / roadmapping peak opacification displays vessels after a test injection allows visualization of live catheter on top to saved image of test injection

Digital Applications Multi-modality imaging / Image fusion PET/CT

DR & Energy Subtraction 2 images taken milliseconds apart at 2 kVp’s Combine / subtract images Soft Tissue Image Bone Image

Other Possibilities Tomosynthesis Histogram Equalization Multi-slice linear tomography from one exposure series Histogram Equalization Use computer to provide approximately equal density to various areas of image.

The End ?