Radiography - The less favoured learning Topic, Yet essential

Image details: An artifact exclusive to digital radiography in the form of Ghosting is  noted 

Teaching points by Dr MGK Murthy

1.Three  types of radiography exist in practice today, film screen variety (conventional) , Computed radiography (CR) using Photo Stimulable Phosphor plate (PSP ) and Digital radiography ( Using Amorphous thin film transistor(TFT) array coupled with phosphor layer of Cesium iodide(CSI) or Gadolinium oxysulfide.)

2.  Additional Artifacts in DR systems  are inherent to technology apart from the patient movement  and  other factors, as do exist in other varieties

3. Two types  amongst them  are important  

(a) Image lag artifact (Ghosting artifact) due to DR design. Ghosting is inability to deactivate detector plate phosphor of the pervious image before the next latent  image  acquisition.

Latent image Signals  in DR systems are readout at regular intervals by application of "Readout" voltage".  However Not  all of the trapped energy may be released by one readout and  residual charge  can be released  during subsequent   readouts of  different exposures of next patient 

This  so called " trapped energy " produces a double exposure kind .

High opaque objects  of the previous image would   reappear as less dense components in the next image  leading to  Ghosting 

This is usually avoided by delaying the next exposure  to at least 60 seconds (can persist up to 15 minutes at times) 

(b) Back scatter artifacts due to inadequate  radiation shielding of  detectors(mostly overcome by major manufactures by placing a thin lead film at the back of the detectors now)

4. Usually the factors of exposure  need to be calibrated and displayed as chart in radiography room  for  all DR systems  to avoid under or over exposures (General  approximate rule is increase 15-20 KvP and 1/3 of mAs of   conventional radiography systems). Standard charts are available from manufactures

5. Three main terms need to be understood for   DR systems (a) Exposure index(EI) (b)  Target Exposure index(EIt)  (c) Deviation Index(DI)

Exposure index is a measure of radiation in relevant image region of the receptor (to be placed by ROI in the anatomical part of intended exposure like chest in chest in CXR and not upper abdomen) 

Target Exposure index is reference exposure obtained when the image is optimally exposed(ideal for the select region) 

Deviation index(DI) is the gap between the two calculated by formula  and hence tells us whether  exposure is optimal for image production (akin to olden days correct  factors and hence readable image)(not for patient dose)

 For example , DI of 1&3 indicate 26% and 100% overexposure and  -1&-3 indicate 20%&50% underexposure 

The exact quantifications of these 3 indices  as indicated by world bodies  like  IEC (international electro technical commission)  and AAPM (American association of Physicists in Medicine) is presently different and efforts on to  find universal measurements  independent of manufacturers 

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