George David Associate Professor of Radiology Medical College of Georgia

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

CR Exposure & Readout

CR Readout

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

Film Screen vs. CR Latitude CR Latitude:.01 – 100 mR 100

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

Digital Radiography (DR) Potentially lower patient dose than CR High latitude as for CR Digital bucky fragile  First DR portables coming to market

Raw Data 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 applied to image prior to presenting it as finished radiograph

Enhancing Raw Image (Image Segmentation) 1.Identify collimated image border 2.Separate raw radiation from anatomy 3.Apply appropriate tone- scale to anatomy LUT  Done with look-up table (LUT) This process is specific to a particular body part and projection *

Look Up Table (LUT) Converts a raw data pixel value to a processed pixel value “Original” raw data pixel value indicates amount of radiation falling on pixel

Image Segmentation Computer must establish location of collimated border of image 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

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

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

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

If adult technique used on peds patients, images look GOOD! If grid removed for ped patient & grid technique used, image looks GOOD NO ONE COMPLAINS CR / DR Latitude More DANGER

Exposure Creep Exposure Creep: Tendency of radiographs toward higher-then-necessary exposures High doses have no detrimental effect on image quality Desire to see less noise on radiographs Increased exposure latitude

So how do I judge the exposure if I can’t tell by looking at the image?

Exposure Index Each manufacturer provides exposure feedback to technologist Displayed on CR reader monitor Displayed on workstations

Exposure Index Measure of radiation received by receptor below anatomy  Not a direct measure of patient exposure If exposure index higher than necessary, patient overexposed

Displayed 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

Exposure Indication Varies between Manufacturers Receptor Exposure Kodak EI Fuji S Number Fuji  “S” number goes down as exposure goes up!  S is half when exposure doubled Kodak  Logarithmic scale  EI goes up 300 when exposure doubled

Exposure Index is Logrithmic EI = * log(exposure) Doubling

Initial Set-up for Exposure Index Kodak recommendation for exposure index: 1800 – 2200 Manual technique:  Technologist should strive to keep exposure index consistent Phototiming:  Set-up by service & physics according to manufacturer’s instructions

Imaging is NOT a Beauty Contest CR/DR exposure should be selected to provide “Maximum tolerable noise”  Keeps dose as low as possible (ALARA) Noise tolerance depends on study & objective. Technologist requires noise feedback from radiologists

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

Fluoroscopy Doses Beam time Geometry Application of features

Last Image Hold Computer displays last fluoro image before radiation shut off. Image noisier than for digital spot  Image made at fluoroscopic technique / intensity  Less radiation than digital spot Allows operator to review static processes while beam off  ideal for teaching  ideal for orthopedic applications such as hip pinning

Fluoro Frame Averaging Computer averages current with user-selectable number of previous frames  Averages current frame & history Conventional fluoro only displays current frame

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

Fluoro Frame Rates Many systems allow option of using lower frame rates  15, 7.5, 3.75 fps rather than 30  computer displays last frame until next one »reduces flicker Most implementations lower patient & scatter exposure  Exposure proportional to frame rate dynamic studies may be jumpy

CT Patient Dose Tube rotates around patient during study Dose distribution different than radiography Patient

USA Today 6/19/2001 “Each year, about 1.6 million children in the USA get CT scans to the head and abdomen — and about 1,500 of those will die later in life of radiation-induced cancer, according to research out today.”

Biggest Question about CT Doses? Appropriateness of Study

Pediatric Color Coding Broselow-Luten Pediatric System Based on child’s size & weight Used in ER’s for  Resuscitation & support apparatus  Medications  IV fluids

Pediatric Color Coding In CT Slightly modified Broselow-Luten Pediatric System Color code is used to determine complex CT protocols  Contrast options  Scanner protocols  Multi-slice has even greater variety of options »Detector configuration »Gantry rotation speed »Table speed Color coding significantly reduces protocol errors

CT Dose depends on kVp mA time pitch filtration noise detector efficiency matrix resolution reconstruction algorithm X-Ray Beam Image Quality & Processing Selections

CT Contrast Resolution Depends on Noise Noise is function of mA

CT Dose Measurement Lucite phantom 5 holes  One center  Four 90 o apart in periphery Chamber placed in one hole Lucite plugs placed in remaining 4 holes Slice centered on phantom Plugs Chamber

Measuring CT Dose Index CTDI “Pencil” ion chamber used Pencil oriented in “Z” direction Dose Phantom Hole for Chamber Chamber Active Chamber Area (exposed area of chamber) Z Z Slice

CTDI 100 Measurement made with 100 mm chamber Includes scatter tails Dose Phantom Chamber Z 100 mm Slice

CTDI W Weighted average of center & peripheral measurements Represents “average” dose in scan plane CTDi P CTDI C CTDI W = 1/3 CTDI 100-center + 2/3 CTDI 100-periphery

Beam Pitch Table motion in one rotation Beam Pitch = Beam thickness Table motion in one rotation Beam Thickness

Beam Pitch Beam Pitch = 1 Beam Pitch > 1 Table motion in one rotation Beam Pitch = Beam thickness

CT Beam Pitch Beam Pitch: 0.75 Beam Pitch: 1.5

Beam Pitch & CT Dose Dose inversely proportional to pitch table motion during one rotation Beam Pitch = Beam thickness Smaller pitch Higher dose Higher pitch Lower dose

Definition of CTDI vol CTDI vol = CTDI w / Pitch Table motion in one rotation Beam Pitch = Beam thickness

Dose Length Product DLP CTDI vol * length of scan (in mGy*cm) Reported on many scanners

CT Dose Tradeoff More dose required to improve noise for same spatial resolution Resolution Noise Dose Noise

CT Dose Reduction Reduce mAs  Increases image noise  Noise inversely proportional to square root of dose Proper technique is maximum tolerable noise as determined by radiologist

CT Noise Reduction: Increase Slice Width more photons detected per voxel BUT More partial volume effect  more different tissue types in each voxel

CT Phototiming Allow operator to specify image quality Noise IndexGE nomenclature: “Noise Index” Modulate mA as tube  rotates around patient  Patient moves through gantry Goal: keep photon flux to detector constant during study

Rotational Beam Modulation Operator specifies maximum mA mA reduced as tube rotates around patient to provide only as radiation as needed

Z-axis Beam Modulation Scanner determines changes in attenuation along z-axis from scout study mA changed as patient moves through gantry

CT Beam Modulation Note more variation in mA during tube rotation in thorax than in abdomen.

CT is High Dose Modality CT head dose fairly uniform For body CT, surface doses approximately 2X dose at center ~ 4 rads ~ 2 rads

Effective Dose “Manufactured” quantity Calculated by multiplying actual organ doses by “risk weighting factors” Units:  Rem »= 1 rad for x-rays  Sievert »= 1 Gray for x-rays

Effective Dose Weighting Factors Tissue / Organ Weighting Factor Gonads.20 Bone marrow.12 Colon.12 Lung.12 Stomach.12 Bladder.05 Tissue / Organ Weighting Factor Liver.05 Esophagus.05 Thyroid.05 Skin.01 Bone Surface.01 Remainder.05

Effective Dose Represents single dose to entire body that gives same cancer risk Allows comparison of different non- uniform exposures 1 mSv (thyroid) 2 mSv (lung) 0.29 mSv (whole body) = (2 mSv X 0.12) + 1 mSv X 0.05) = 0.29 mSv

Effective Doses for Various Studies

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