Method to measure the entrance dose of a radiology apparatus

The invention relates to a method to measure the entrance dose of a radiology apparatus of the type comprising an X-ray beam source, and a detector for the X-ray beam after it has passed an object of interest carried on a table placed between the source and the detector, wherein the X-ray beam source is placed above said table, and wherein the distance between the X-ray beam source and the object is estimated taking into account the object's morphology, for determining said entrance dose.

Such a method is known from EP-A-1 035 420.

With such a method it is intended to derive an accurate value of the actual X-ray dose received by the object, usually a patient. The importance hereof is self-evident.

The invention aims to provide a simple, yet reliable method to take the object's morphology into account for determining said entrance dose.

In this respect the method according to the invention is characterized in that an attenuation-ratio is calculated depending on energy-levels of the X-ray beam leaving the X-ray beam source and arriving at the detector respectively, and that the object's thickness is calculated depending on said attenuation-ratio, said thickness together with the table-position defining said distance between the X-ray beam source and the object.

Once the source to object distance is known, the entrance dose of the X-ray beam arriving at the patient can easily be calculated in accordance with applicable methods known from the prior art.

The ease of calculating the entrance dose of the X-ray beam arriving at the patient is supported by the measure that the energy-level of the X-ray beam leaving the X-ray beam source is calculated with a model-description of said X-ray beam source. Such a model description of the X-ray beam source is known per se from the prior art such as for instance U.S. Pat. No. 4,158,138.

It is preferable that the object's thickness is calculated depending on the attenuation-ratio, and further taking into account at least a first correction factor pertaining to the region of interest of the concerning object. In this manner the determination of the object's thickness can be maintained at the required level of accuracy, although the investigated part of the patient more than moderately deviates from the normally investigated waterlike tissue of such patient. In this way also head, chest and pelvis may be suitably investigated.

It is further preferred that in calculating the object's thickness a second correction factor is used relating to ageing effects of the X-ray beam source. This may be implemented in line with regular calibrations of the concerning radiology apparatus that is used.

Of course, special precautions have to be taken in the case of application of shutters, wedges and filters. When these objects get into the measuring field, their position and attenuation-values must be known and consequently taken into account when deriving the patient's thickness. Special precautions should be taken to prevent unnoticed application of external filters in the filter holding slits of the collimator housing of the apparatus.

The invention also relates to a radiology apparatus. The radiology apparatus according to the invention as defined in claim 5 is able to operate according to the method of the invention. The invention also pertain to a computer programme. The computer programme according to the invention is defined in claim 6 . When the computer programme of the invention is loaded into the working memory of a processor that is comprised in a radiology apparatus to control the operation of the radiology apparatus, this enables the radiology apparatus to carry out the method of the invention.

The invention will hereafter be further elucidated with reference to the drawing showing a single FIGURE.

The FIGURE is a schematic view of a radiology apparatus used in the method of the invention.

FIG. 1 shows that the radiology apparatus comprises an X-ray beam source 1 which is movable in height as shown by arrow 15 .

The X-ray beam source 1 is placed above a table 10 on which a person 5 who is to be monitored is lying. Under the table 10 detector 6 is placed for receiving the X-ray beam transmitted by the X-ray beam source 1 after it has passed the body of the person 5 .

In order to measure the entrance dose at the skin of the person 5 an attenuation-ratio is calculated that depends on the energy-levels of the X-ray beam leaving the X-ray beam source 1 and arriving at the detector 6 respectively.

Said attenuation-ratio is used to estimate the person's 5 thickness and this thickness together with the actual position of the table 10 vis-à-vis the X-ray beam source 1 is used to determine the distance between the X-ray beam source 1 and the person 5 .

Obviously the apparatus as shown in FIG. 1 can also be implemented such that the X-ray beam source 1 is stationery and that the table 10 can be moved in the vertical direction to a certain desired height level.

In a preferred embodiment of the invention the energy-level of the X-ray beam leaving the X-ray beam source 1 is calculated with a predetermined model description of the X-ray beam source 1 .

It is further preferred that depending on the part of the body of the person 5 that is being monitored a correction factor is applied in the derivation of the person's 5 thickness depending on the measured attenuation-ratio.

Furthermore, it is desirable that a second correction factor is used to take into account ageing effects of the X-ray beam source 1 .