Creating a Technique Chart Creating a Technique Chart for the Canine Thorax for the Canine Thorax

Optimizing Image QualityOptimizing Image QualityInherent subject contrastInherent subject contrast

Thorax has high inherent subject Thorax has high inherent subject contrast c/f abdomencontrast c/f abdomenPrimarily air and soft tissuePrimarily air and soft tissueLooking for subtle changes in opacityLooking for subtle changes in opacityNeed a long Need a long ‘‘gray scalegray scale’’ = wide latitude = wide latitude Patient motion is a major concern Patient motion is a major concern

Optimizing Image QualityOptimizing Image Qualityfilm / screen systemsfilm / screen systems

Motion and contrastMotion and contrastHigh detail not critical (ex cats)High detail not critical (ex cats)System speed and latitude more System speed and latitude more important important Use a detail system when Use a detail system when ‘‘table toptable top’’Use a grid when patient thicker than Use a grid when patient thicker than 44”” with a 250 speed or higher systemwith a 250 speed or higher system

Optimizing Image QualityOptimizing Image QualityMachine SettingsMachine Settings

Maximize kVpMaximize kVpMaximize mA setting Maximize mA setting Minimize exposure time Minimize exposure time Use kVp variable technique chartsUse kVp variable technique charts

High kVp affords High kVp affords ‘‘latitudelatitude’’ and reduces and reduces the total the total mAsmAs required for optimal required for optimal

film blackness film blackness

For the same film blackness

If halve mAs, increase kVp by 20%

If double the mAs, decrease kVp by 16%Note:

Increasing kVp will cause an increase in film blackness due to both higher photon #

and higher photon penetration (higher photon energy = less patient attenuation)

kVp and mAs RelationshipkVp and mAs Relationship

Setting kVp and mAsSetting kVp and mAsExcessive film backness (overexposed)

kVp or mAs or both too high

Insufficient film blackness(underexposed)kVp or mAs or both too low.

Note:Insufficient mAs = gray blotchy background

often called ‘quantum mottle’In veterinary medicine we tend to preset the mAs

and adjust kVp based on patient dimensions

Tissue thickness / kVp Tissue thickness / kVp relationship (per cm change)relationship (per cm change)

Up to 80 kVp +/- 2kVp80 - 100 kVp +/- 3kVp100+ kVp +/- 4 kVp

This is the basis of making variable kVp technique charts.

Factors that affect image Factors that affect image contrastcontrast

Inherent subject contrast kVp and beam filtration (major variable)Film / screen systemGrid (scatter)Darkroom setup and technique

Selecting an appropriate kVp Selecting an appropriate kVp range range

Thoracic radiographyUse a high kVp technique- more inherent subject contrast- subtle changes in lung parenchyma

Abdominal radiography Use a lower kVp technique- less inherent subject contrast- Looking for gross organ identification

Selecting mAsSelecting mAsMotion blur is a significant problem in Veterinary Radiography (machine specs)Always attempt to minimize exposure time (s) so as to minimize movement artifactAlways use maximum mA setting generator will allow

(mA x s = mAs)

Thorax Technique Chart Thorax Technique Chart Select appropriate film/screen system Measure patient at widest point (liver) and record lateral and dorsoventraldimensions (cm)Determine what grid is present Select a ‘base’ mAs – suggest 2.5 mAsCreate 4 exposures – 50, 60, 75 and 90 kVp of lateral thorax at

peak inspiration

Assess which exposure closest to optimum Make 2 additional 3 exposures around the value selected (+/- 4 keV)Select the optimum exposure. (extrapolate if necessary) Generate a technique chart from 10cm to 30 cm using the std kVp /cm relationshipTake a VD view using the technique chart and assess quality

Effect of Grid Effect of Grid Use the standard technique chart to generate a good quality abdominal radiograph Radiograph the patient again using a table top technique with the same exposure Radiograph the patient again – but reduce the mAs by half and reduce the kVp by 16%

Explain what you see….