## Mapping Grey Levels to Optical Density for the HOWTEK Scanner at MGH

Cases with a filename
beginning with the letter 'A' came from MGH. There were two scanners used at MGH. Cases
that have HOWTEK in the .ics file were scanned on the HOWTEK scanner at MGH and can be
calibrated to optical density with the equations in this file.

A step-wedge phantom film was scanned on the HOWTEK scanner at the full 43.5 micron
per pixel spatial resolution. A rotated, 8x aggregated version of the scanned
step-wedge is displayed below.

The optical density of each step was measured several times, and the average
values were calculated. The step-wedge was scanned on six days over a 16 day
period. The mean and the standard deviation was then calculated for each
step in each (43.5 micron) scanned image. The average of the 6 means and of the
6 standard deviations were then computed for each step.
The results are listed below:

Measured Grey Level Mean
Optical Density (Standard Deviation)
--------------- --------------------
3.0600 820 (67.75)
2.9067 973 (49.15)
2.7433 1117 (33.41)
2.6033 1267 (29.57)
2.4500 1425 (24.51)
2.3033 1565 (19.21)
2.1500 1710 (18.81)
2.0000 1863 (17.52)
1.8400 2036 (13.33)
1.6900 2200 (12.65)
1.5467 2355 (10.80)
1.3967 2498 (10.69)
1.2400 2663 (11.72)
1.0933 2830 ( 9.38)
0.9367 2990 ( 9.09)
0.7900 3158 (10.83)
0.6300 3341 ( 9.11)
0.4800 3501 ( 9.37)
0.3300 3681 (10.11)
0.2100 3826 (12.25)
0.0400 4012 ( 4.96)

Here is a plot of mean grey level versus the measured optical density of
the step wedge phantom.

Using simple regression to fit a line to the data, we get an intercept of 4005.7
and a slope of -1056.6.

So, to convert a grey level (GL) value to optical density (OD), simply use the
following equation:

**Eq. 1** | OD = 3.789 - 0.00094568 * GL |

A plot showing the Optical Density (OD) vs. Gray Level (GL) is given below. The
x's show sample data and the line shows a plot of **Eq 1**.