Parameter

Air Volume Chamber

Solid State Detector

Handling

Air volume chambers are very sensitive to rough handling. Dents in the chamber surface can change the volume of the chamber, which can affect its calibration.

Digital dose probes are generally well protected from physical damage

Touching the air chamber portion of the measuring probe during a measurement can affect the reading.

Solid state detectors can tolerate being touched or placed under phantoms for measurements.

Air volume chambers often use 150 - 300 Volt bias voltages, so be certain that all power is off before swapping probes.

No bias voltages are involved, but "hot swapping" probes is not recommended.

Air volume probes are generally calibrated at a specified room temperature and pressure. Newer meters may be able to compensate for environmental changes, but older meters (such as the Radcal 1515) have to be manually corrected to ensure maximum accuracy.

Meter and probes should be allowed to normalize to room temperature before measurements are taken.

Can be effected by EMF and other field potentials.

Meter can be affected by EMF and other field potentials. This often manifests itself by spurious readings, of tiny numeric value

The volume of the air chamber determines the quantity of molecules that can be ionized to create a measurable charge or current. Larger volume meters contain more molecules, which mean lower fluence rates can produce a measurable signal.

The PN junction that forms the detector has limited number of electrons which may be ionized.

In analog meters, the input circuitry must balance gain (which amplifies noise) with current/charge capacity.

The expert user must select a probe with sensitivity and range within the fluence rates expected for the test being performed.

Amplification is often automatic, with manual settings optional.

The detector probe is calibrated according to beam energies, and HVL. The PMX meter uses ADI's, (Automatic Detector Identification modules), which uses EEPROMs to store calibration data. Standard ADI's are for Mammography and Diagnostic imaging energies. The (2-d)ADI is for Mammography x-ray beams in the 23 - 35kV range, originating from a molybdenum target and filtered by a molybdenum filter.

Probes for measuring patient exposures for Diagnostic radiography and fluoroscopy are about 6 cc.

The (2-b) ADI is for Diagnostic x-ray beams in the 50 - 150kV range, originating from a tungsten, or tungsten-rhenium target, and filtered by 3mm of Aluminum

Probes to measure leakage and scatter must be, by MOH regulation, of no less surface area than 100 cm² , with no linear dimension greater than 10cm. For the RadCal line, this is an 180cc chamber (pancake or flat probe).

For extremely sensitive measurements, an 1800cc probe is available.

Standard PMX detectors are not suitable for measuring leakage, or scatter and a special detector must be used for measuring patient exit exposures.

Diagnostic radiography probes are NOT to be used on mammography. The aluminum cover absorbs far too many x-rays to give an accurate value. A special beryllium windowed probe, calibrated for energies between 23 - 35kV must be used for mammography.

Use of the ADI modules for x-ray beams that do not have the beam characteristics given above can result in erroneous measurements. Copper, and rare earth filters for diagnostic energied beams can cause erroneous readings, as will Rhodium filtration or Rhodium or Tungsten targets for Mammography. If a specific filter/target combination is not covered by the existing ADI, take the measurement "AS IS", and record the filter/target specifics, so that correction to the raw data may be made in the future, if necessary.

Saturation

When the maximum number of ions for an air volume are created, further increase in the fluence rate will not be reflected in the measurement. Also, if a x-ray beam releases more charge or current than the input amplifiers of the meter can handle, the meter saturates. Normally, the second condition produces an error code, but the user is warned to always check the appropriateness of probe for testing situation.

When the maximum number of electrons in the PN junction have been released, any further increase in the fluence rate will not be reflected in the measurement. If the x-ray beam produces more electrons than the input amplifiers gain setting range, the meter will saturate. Generally, a MX detector at more than 50cm from a tube focus, for kV's under 125, and mA's less than 800 should not saturate. If in doubt, move the meter up and down, and see if the measurement changes.

Insufficient coverage

When a probe's volume is not completely exposed to the beam being measured, a reduced charge or current will be produced, leading to erroneously low readings. Always ensure that the probe is fully in the beam.

The same applies for solid state detector probes, but because of their small size, it is fairly difficult to under exposure the measuring surface.

Filtration Dependence

Correction tables are available to compensate for non-standard beams. Generally, no dose rate correction is applied for beam filtrations between 3 - 5 mm Al HVL at 80kV

Probes can be calibrated for any specific HVL, by the OEM.

Correction tables are available to compensate for non-standard beams. These tables are automatically applied to measurements taken with the oRTIgo software. For stand alone testing, no corrections are usually applied for beam filtrations between 3 - 5 mm Al HVL at 80kV. THIS SHOULD BE INDICATED ON THE TEST FORM.

A special detector can be purchased for ultra high HVLs, and it is used for measuring II entrance exposures (patient exit exposure).

The OEM can calibrate the detectors for special HVL's, such as heavy copper or rare earths.

Backscatter sensitivity

Most Radiographic exposure measurements should be made in air, without backscatter. When doing measurements, it is important to protect the probe from backscatter, or else false high readings can be obtained.

For fluoroscopy, and CINE dosimetry, backscatter must be included in the reading, and the probe must be positioned to pick up the scatter, or a back scatter correction must be applied.

The solid state detector is normally manufactured with a lead backing, to prevent backscatter from affecting measurements.

This lead strip prevents fluoro measurements from including backscatter, so a correction to the measurement must be done. Based on an average correction, we multiply the reading by 1.3.

WARNING: THE LEAD StrIP CAN NOTICABLY AFFECT AUTOMATIC DOSE CONtrOL FOR SMALL FIELD MEASUREMENTS. try to keep only the tip of the probe in the field when making measurements.