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Technique Optimization in Digital Mammography

 

Dose reduction when using optimal W-Rh technique compared to reference clinical Mo-Mo technique

 

 

 
   

Breast  Phantom   Composition

  Breast thickness (cm)

Adipose

Mixed
(50-50)

Glandular

  2 9% (12%) 12% (16% ) 26% (30%)
  4 26% (29%) 39% (41%) 52% (55%)
  6 40% (41%) 51% (49%) 62% (63%)
  8 57% (55%) 57% (54%) 38% (41%)
 

The results reported are for masses and calcifications (calcification results are shown in parentheses) Mass and calcification dose savings agreed to within 1-4%.


The increased use of digital mammography has renewed the issue of optimizing the technique to achieve maximum image quality at the lowest achievable applied dose.

Our recent work in this area has evaluated the dose savings possible by switching from analog to digital. Since it is the intrinsic performance of imaging systems, or the signal to noise of the system in relation to applied dose, that is a key predictor of image quality, we established a criteria for assessing the impact of technique (such as kVp settings, or target/filter combinations) on image quality by employing a figure of merit (FOM) equal to the signal-difference to noise ratio squared (SdNR2) normalized by mean glandular dose (MGD).

This FOM was evaluated for both Mo-Mo and W-Rh target/filter combinations for  breast  thicknesses  of 2 - 8 cm with various glandular compositions, using a clinical a-Se full-field digital system from Siemens Healthcare. Based on the FOM results, we assessed the dependency of system performance on spectral quality to draw conclusions regarding the achievable dose savings in the migration to new optimized techniques specific to digital mammography.

 

 
   
Top view of the relative position of mass and calcium simulated lesions with respect to the breast phantoms.  Masses are shown in light grey, calcifications are shown in dark grey.  Section 1 is 100% adipose; section 2 is 50% adipose and 50% glandular; section 3 is 100% glandular.
 
   

The results of our study have demonstrated that significant dose savings (on the order of 9% - 63%) could be achieved with the use of optimized W-Rh spectra in comparison with pre-existing clinical techniques using Mo-Mo, demonstrating the importance of optimizing technique for use with digital mammography systems. Our study results also suggest that the optimization of technique is independent of lesion type, i.e. there is no need to compromise the visualization of masses over calcifications, or vice versa.

Studies to correlate conventional technique optimization with other optimization approaches, ie. evaluating effective detective quantum efficiency normalized by effective dose as an indicator of mammographic image quality are ongoing. Future studies expanding our optimization regimen are planned including investigation of novel target/filter combinations to determine the optimal technique for evaluating breasts in the upper range of density and thickness.



Technique variables employed for this evaluation

 

Study Parameter

 

Number of Variables

Description

 
Target/Filter 2 Mo-Mo, W-Rh
kVp settings 6 23, 26, 28, 30, 32, 35
Breast Phantom Composition 3 100% adipose,
50% fat / 50% glandular,
100% glandular
Breast thickness 4 2, 4, 6 and 8 cm
Inclusions (mass and calcification) 2 with, without




 


Figure of Merit as a Function of kVp for

two Target/Filter Combinations

 

 
 

Figure 2: Graphs showing the relationship between the figure of merit (FOM) and kVp for both Mo-Mo and W-Rh target/filter combinations for 100% adipose (A,D), 50% adipose / 50% glandular (B,E), and 100% glandular (C,F). The FOM, defined by the ratio of SdNR2/MGD was calculated for both mass (A,B,C) and calcification (D,E,F) lesions, where SdNR is the signal-difference-to-noise ratio and MGD is the mean glandular dose. The solid grey circles represent the previously-established clinical technique based on Mo-Mo and screen/film, shown here for comparison purposes.

 
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