Density control in radiographs – ASME V

By Carlos F Molina

In this article we will be talking more on the subject of radiology in the ASME V code

NOTE: This article is half of what originally was intended to be. It turns out that, as usual, good research takes a lot of time and as consequence, articles end up being longer and not reader-friendly.  So I trimmed out the info and I am resizing with the info needed by API 653 readers.

There is 1 characteristic of a good radiograph that you should control:

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FILM DENSITY

According to ASME V,

either (1) a densitometer or (2) step wedge comparison film shall be used for judging film density.

Radiographic density  is a measure of the degree of film darkening.  Technically it should be called “transmitted density” when associated with transparent-base film since it is a measure of the light transmitted through the film.  Radiographic density referred to as the overall blackening of the film. Density is dependent on amount of radiation received by the film, that depends, among other things, on source size and time of exposure.

DENSITOMETERS

A densitometer is a device that measures the amount of light transmitted through an object, in this case a piece of radiographic film. The film is placed between the light source and the sensor and a density reading is produced by the instrument. SIDE NOTE: Have you seen the movie “The pursuit of happyness”? The main charachter  sells x-ray bone density scanners (didn´t happen in real life though) that really looked like the one in this image

DENSITOMETERS NEED TO BE CALIBRATED

Densitometers shall be calibrated at least every 90 days during use as follows:
(a) A national standard step tablet or a step wedge calibration film, traceable to a national standard step tablet and having at least 5 steps with neutral densities from at least 1.0 through 4.0, shall be used. The step wedge calibration film shall have been verified within the last year by comparison with a national standard step tablet unless, prior to first use, it was maintained in the original lighttight and waterproof sealed package as supplied by the manufacturer. Step wedge calibration films may be used without verification for one year upon opening, provided it is within the manufacturer’s stated shelf life.
(b) The densitometer manufacturer’s step-by-step instructions for the operation of the densitometer shall be followed.
(c) The density steps closest to 1.0, 2.0, 3.0, and 4.0 on the national standard step tablet or step wedge calibration film shall be read.
(d) The densitometer is acceptable if the density readings do not vary by more than ±0.05 density units from the actual density stated on the national standard step tablet or step wedge calibration film.

When the code says “A national standard step tablet”, it is likely talking about a NIST  step tablet. Before, NIST created the SRM-1001 step tablet with 17 steps (now 38100C, info for SRM-1001 can be found here) and  the SRM-1008 step tablet with 23 steps (now 38120C, info for SRM-1008 can be found here). The instructions on the NIST site state the following regarding densitometer calibration

INSTRUCTIONS FOR USE

Remove the film from its protective sleeve and place the center of a step on the diffuser of the densitometer to be calibrated with the side of the step tablet with the serial number in contact with the diffuser. Calibrate the densitometer using the transmission density of that step, and repeat this procedure with other steps of the step tablet.

Of course, NIST is for US, and most of other countries refer to US standards. For germany, it exists Physikalisch -Technische Bundesanstalt PTB standard reference X-ray Fil step tablet – calibration mark 3641-02. The national standard step tablet shall be stored properly in between every 90-day calibration.

Step wedge calibration films are processed films with discrete density steps that have been verified by comparison with a national standard step tablet. They can be used for calibration.

STEP WEDGE COMPARISON FILM SHOULD BE VERIFIED

Step Wedge Comparison Films. Step wedge comparison films shall be verified prior to first use, unless performed by the manufacturer, as follows:
(a) The density of the steps on a step wedge comparison film shall be verified by a calibrated densitometer.
(b) The step wedge comparison film is acceptable if the density readings do not vary by more than ±0.1 density units from the density stated on the step wedge comparison film.
Periodic Verification.
(a) Densitometers. Periodic cablibration verification checks shall be performed as described in T-262.1 at the beginning of each shift, after 8 hr of continuous use, or after change of apertures, whichever comes first.
(b) Step Wedge Comparison Films. Verification checks shall be performed annually per T-262.2. T-262.4 Documentation.
(a) Densitometers. Densitometer calibrations required by T-262.1 shall be documented, but the actual readings for each step do not have to be recorded. Periodic densitometer verification checks required by T-262.3(a) do not have to be documented.
(b) Step Wedge Calibration Films. Step wedge calibration film verifications required by T-262.1(a) shall be documented, but the actual readings for each step do not have to be recorded.
(c) Step Wedge Comparison Films. Step wedge comparison film verifications required by T-262.2 and T-262.3(b) shall be documented, but the actual readings for each step do not have to be recorded.

Step wedge comparison films are processed films with discrete density steps that are to be verified by use of a calibrated densitometer before first use, and that is used to determine if production radiographs meet density limits. This step wedges are needed fo the verifications of densitometers that are to be made before and after each use, after every 8 hours of continuous use, and so. Step wedge comparison films need not be calibrated, mostly they are a way to compare a densitometer against itself.

At least once a year, the densities of comparison films are re-measured against the national standard step tablet and the recorded values adjusted as necessary. During these frequency checks, if the readings from the densitometer do not match the recorded densities from the reference comparison film, then you verify the densitometer against the step tablet.

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CONCLUSION

Next week we re going to talk about radiographic techniques and allowable densities in radiograph, and that will be the end of our review on radiographs according to ASME V. Thanks four your reading.