Fundamentals of Digital Radiology (year 1) George David, MS, FAAPM, FACR Medical College of Georgia

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

What we really mean by Digital No more File Room!!!

Digital Image Formation Place mesh over image

Digital Image Formation Assign each square (pixel) a number based on density Numbers form the digital image 194 73 22

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

Numbers / Gray Shades Each number of a digital image corresponds to a gray shade for one picture element or pixel

Computer Storage 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 Storage = 262,144 pixels X 1 byte / pixel = 262,144 bytes = 256 KBytes = .25 MBytes

Image Size Related to both matrix size & bit depth higher (finer) matrix requires more storage doubling matrix size quadruples image size higher bit depth requires more storage doubling bit depth theoretically doubles image size Computer may require storage in multiples of 8 bits (bytes) 10 or 12 bits stored in 16 bit slot alters image size requirements 1 2 3 4 5 6 7 8 9

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

So what is a digital image? Image stored as 2D array of #’s representing some image attribute such as optical density x-ray attenuation echo intensity magnetization 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

Computer Storage 125 25 311 111 182 222 176 199 192 85 69 133 149 112 77 103 118 139 154 120 145 301 256 223 287 225 178 322 325 299 353 333 300 125, 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

= Digital Copies Digital copies are identical All digital images are originals =

Image Matrix 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

Matrix Size & Resolution More pixels = better spatial resolution

The Bit Fundamental unit of computer storage

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

Bit Depth & Contrast Resolution bit depth indicates # of possible brightness levels for a pixel 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 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 Storage = 262,144 pixels X 1 byte / pixel = 262,144 bytes = 256 KBytes = .25 MBytes

Image Size Related to both matrix size & bit depth higher (finer) matrix requires more storage doubling matrix size quadruples image size higher bit depth requires more storage doubling bit depth theoretically doubles image size Computer may require storage in multiples of 8 bits (bytes) 10 or 12 bits stored in 16 bit slot alters image size requirements 1 2 3 4 5 6 7 8 9

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

CR Operation after read-out, plate erased using a bright light plate can be erased virtually without limit Plate life defined not by erasure cycles but by physical wear

Another View: CR Operation

Computer Radiography (CR) plate is photostimulable phosphor radiation traps electrons in high energy states higher 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 reader scans plate with laser laser releases electrons trapped in high energy states electrons fall to low energy states electrons give up energy as visible light light intensity is measure of incident radiation Lower Energy Electron State

Reading Imaging Plate Reader scans plate with laser light using rotating mirror Film pulled through scanner by rollers Light given off 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 color of light emitted by phosphor measured by PMT

CR Operation after read-out, plate erased using a bright light plate can be erased virtually without limit Plate life defined not by erasure cycles but by physical wear

CR Throughput Generally slower than film processing CR reader must finish reading one plate before starting to read 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 100

Digital Radiography (DR) Digital bucky Electronic Incorporated into x-ray equipment

Digital Radiography (DR) Receptor provides direct digital output No processor / reader required Images available in ~ 5 seconds Much faster throughput Eliminates many steps for technologist

Raw Data Image Unprocessed image as read from receptor 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 defines anatomic region Computer establishes collimated border of image Computer defines anatomic region Finished image produced by tone scaling Requires histogram analysis of anatomic region

Image Post-Processing Histogram Graph showing how much of image is exposed at various levels Tone scaling Body part & projection-specific algorithms Must correctly identify anatomical region Histogram used to display image with proper Density Contrast

Film/Screen Limited Latitude Film use has little ambiguity about proper radiation exposure

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

So how do I know if exposure is optimum by looking at my image?

Exposure Index Each manufacturer provides feedback to technologist on exposure to digital receptor Displayed on PACS monitor

Calculated Exposure Index Affected by X-Ray technique selection Improper centering of image on cassette Improper selection of study or projection Placing two or more views on same cassette Can cause image to appear dark

Phototimed 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

Last Image Hold Computer displays last fluoro image Allows operator to review static processes without keeping beam on

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

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 Possibilities 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

DR Mobile Units GE Definium AMX 700 See image immediately Wireless transmission of images GE Definium AMX 700

The End ?