Apparatus for and method of capturing radiation image

Before a radiation image is captured, a rate of change of an output signal from an integrator, during a given period of time after the integrator has been reset and until a radiation start signal is supplied, is calculated. An offset voltage signal at a desired time is calculated using the rate of change, and is supplied to a voltage correcting circuit. An output signal from the integrator after a radiation X has started being applied to a subject is corrected based on the calculated offset voltage signal. The corrected output signal from the integrator is supplied to an X-ray tube controller for controlling application of the radiation X to the subject.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for and a method of capturing a radiation image by emitting radiation from a radiation source and applying the emitted radiation to a subject, and for controlling the radiation source based on the dose of radiation applied to the subject.

2. Description of the Related Art

In the medical field, for example, it has been customary to apply radiation to a subject and to detect the amount of radiation that has passed through the subject with a radiation detector, or to guide the radiation that has passed through the subject directly to an X-ray film or the like, thereby forming a radiation image of the subject for diagnostic purposes.

For obtaining a radiation image suitable for image interpretation and diagnosis, a tube voltage, a tube current, and a radiation application time are established as appropriate exposure conditions, depending on the body region to be imaged and other characteristics of the radiation source. There is known an image capturing system with an automatic exposure control function, which controls the dose of radiation to be applied to a subject, by detecting the dose of radiation that has passed through the subject with a dose detector, and then automatically stopping application of radiation when the detected dose reaches a predetermined value (see Japanese Laid-Open Patent Publication No. 2004-298383).

In the image capturing system having such an automatic exposure control function, a small current output from the dose detector is converted into a voltage signal by a current-to-voltage converter. The voltage signal is amplified at a high magnification by an amplifier, and the amplified voltage signal is integrated with respect to time by an integrator, thereby determining a radiation dosage.

However, since the radiation dosage is determined after a low voltage signal has been amplified at a high magnification, the image capturing system having such an automatic exposure control function is problematic in that a temperature-dependent characteristic change of the circuit components tends to cause a large error in the determined radiation dosage.

In order to compensate for such temperature-dependent characteristic changes of the circuit components, there has widely been employed a process of canceling out the characteristic circuit component changes using a temperature compensating device, whose input/output characteristics change depending on temperature, such as a thermistor, a diode, or the like, wherein the temperature compensating device is inserted into the system circuitry (see Japanese Laid-Open Patent Publication No. 5-87607 and Japanese Laid-Open Patent Publication No. No. 5-299955).

The temperature characteristics of the temperature compensating device vary from unit to unit. Therefore, if the temperature compensating device is not thermally coupled adequately to the circuit component whose temperature-dependent characteristic change is to be compensated for, then the temperature compensating device cannot provide highly accurate temperature compensation. In addition, the circuit component has its own time-variable characteristics, and as a practical matter, it is difficult to select a temperature compensating device that is capable of handling variations in characteristics.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an apparatus for and a method of capturing a radiation image while highly accurately determining the radiation dosage, so as to control a radiation source regardless of temperature-dependent and time-dependent characteristic changes in the circuit components thereof.

A major object of the present invention is to provide an apparatus for and a method of capturing a radiation image by applying an appropriate radiation dosage to a subject regardless of temperature-dependent and time-dependent characteristic changes in the circuit components thereof.

With the apparatus for and method of capturing a radiation image according to the present invention, before radiation is applied to a subject to capture a radiation image of the subject, a rate of change of an output value provided from a radiation dosage calculating unit is determined. An offset value, for correcting the output value of the radiation dosage calculating unit, is determined using the rate of change. A radiation dosage calculated by the radiation dosage calculating unit, or a preset required radiation dosage, is corrected based on the offset value, in order to appropriately control the radiation source so as to capture a radiation image, regardless of characteristic changes in the circuit components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows in perspective a mammographic system12to which an apparatus for and method of capturing a radiation image according to an embodiment of the present invention are applied.

As shown inFIG. 1, the mammographic system12has an upstanding base26, a vertical arm30fixed to a horizontal swing shaft28disposed substantially centrally on the base26, a radiation source housing unit34storing a radiation source for applying radiation to a subject32and which is fixed to an upper end of the arm30, an image capturing base36housing a solid-state detector for detecting radiation that has passed through the subject32and which is fixed to a lower end of the arm30, and a presser plate38for pressing and holding the subject's breast against the image capturing base36.

When the arm30, to which the radiation source housing unit34and the image capturing base36are secured, is angularly moved about the swing shaft28in a direction indicated by the arrow A, an image capturing direction with respect to the breast of the subject32can be adjusted. The presser plate38is connected to the arm30and is disposed between the radiation source housing unit34and the image capturing base36. The presser plate38is vertically displaceable along the arm30in a direction indicated by the arrow B.

To the base26, there are connected a control panel40for entering image capturing information including ID information of the subject32, an image capturing region of the subject32, a tube voltage, a target type, the type of filter to be mounted in an opening58(seeFIG. 2) for adjusting the radiation dose, etc., and a display panel39for displaying the entered image capturing information.

The display panel39and the control panel40may be mounted on a console (not shown) connected to the mammographic system12, rather than being mounted on the mammographic system12itself.

FIG. 2shows internal structural details of the radiation source housing unit34.

As shown inFIG. 2, the radiation source housing unit34has a target54serving as a radiation source made of molybdenum, tungsten, or the like, which is placed in a housing52, and a cathode56for emitting an electron beam “e” to the target54. The housing52has an opening58defined in a lower wall thereof through which radiation X, which is generated when the electron beam “e” emitted from the cathode56bombards the target54, passes toward a breast44of the subject32that is to be imaged within a predetermined exposure field. The housing52is made of a heavy metal for preventing radiation from leaking outside of the housing52.

FIG. 3shows internal structural details of the image capturing base36of the mammographic system12. InFIG. 3, the breast44of the subject32is shown as being placed between the image capturing base36and the presser plate38.

The image capturing base36houses therein an X-ray detector46for detecting the radiation X that is emitted from the target54of the radiation source housing unit34through the opening58, an exposure control sensor47(dose detecting means) for detecting the dose of radiation X that has passed through a desired region of the breast44for performing exposure control, a reading light source48for applying a reading light to the X-ray detector46so as to read information of the radiation X that is detected by the X-ray detector46, and an erasing light source50for applying an erasing light to the X-ray detector46in order to remove unwanted electric charges accumulated within the X-ray detector46.

The X-ray detector46comprises a direct-conversion light-reading radiation solid-state detector. The X-ray detector46stores information of the radiation X that has passed through the breast44as an electrostatic image, and when the X-ray detector46is scanned by reading light applied from the reading light source48, generates a current depending on the electrostatic image.

More specifically, the X-ray detector46comprises a laminated assembly made up of a first electrically conductive layer disposed on a glass substrate for passing the radiation X therethrough, a recording photoconductive layer for generating electric charges upon exposure to the radiation X, a charge transport layer which acts substantially as an electric insulator with respect to latent image polarity electric charges developed in the first electrically conductive layer, and further which acts substantially as an electric conductor with respect to transport polarity charges, which are of a polarity opposite to the latent image polarity electric charges, a reading photoconductive layer for generating electric charges and which becomes electrically conductive upon exposure to the reading light, and a second electrically conductive layer which is permeable to the radiation X. An electric energy storage region is provided within an interface between the recording photoconductive layer and the charge transport layer.

The first electrically conductive layer and the second electrically conductive layer each provides an electrode. The electrode provided by the first electrically conductive layer comprises a two-dimensional flat electrode. The electrode provided by the second electrically conductive layer comprises a plurality of linear electrodes, which are spaced at a predetermined pixel pitch, for detecting the information of the radiation X that is to be recorded as an image signal. The linear electrodes are arranged in an array along a main scanning direction, and extend in an auxiliary scanning direction perpendicular to the main scanning direction.

The reading light source48has, for example, a line light source comprising a linear array of LED chips and an optical system for applying a line of reading light emitted from the line light source to the X-ray detector46. The linear array of LED chips extends perpendicularly to the direction in which the linear electrodes of the second electrically conductive layer of the X-ray detector46extend. The line light source moves along the direction in which the linear electrodes extend so as to expose and scan the entire surface of the X-ray detector46.

The erasing light source50should preferably comprise a light source, which can emit and quench light within a short period of time and which has very short persistence. For example, the erasing light source50may comprise a plurality of external-electrode rare-gas fluorescent lamps, extending along the direction of the array of LED chips of the reading light source48, and arranged in an array perpendicular to the direction of the array of LED chips of the reading light source48.

The exposure control sensor47comprises a photodiode or the like, which is movable to a desired position between the X-ray detector46and the erasing light source50, for detecting the dose of radiation X that has passed through the desired region of the breast44. The desired region of the breast44may be the mammary gland region, for example.

FIG. 4shows in block form a control circuit of the mammographic system12.

As shown inFIG. 4, the mammographic system12includes a parameter memory60for storing various parameters such as the absorption rates of different regions to be imaged at which the subject32absorbs the radiation X, the absorption rate at which the presser plate38absorbs the radiation X, the sensitivity of the X-ray detector46, an index that depends on the atomic number of the target54, the attenuation characteristics of the radiation X depending on the distance between the target54and the X-ray detector46, etc. The mammographic system12further includes a subject thickness measuring unit62for measuring from positional information of the presser plate38a subject thickness, i.e., the thickness of a region to be imaged, and an exposure condition setting unit64for setting exposure conditions including a tube current, a radiation application time, a radiation dosage, etc., using image capturing information representing the region to be imaged of the subject32, a tube voltage, the target54and the filter types that have been entered from the control panel40, parameters read from the parameter memory60, and data of the subject thickness supplied from the subject thickness measuring unit62. In addition, the mammographic system12includes an X-ray tube controller68for controlling an X-ray tube66, which comprises the cathode56and the target54of the radiation source housing unit34, according to exposure conditions set by the exposure condition setting unit64, a radiation image forming unit70for forming a radiation image of the breast44based on information of the radiation X detected by the X-ray detector46, and an exposure controller72for calculating a radiation dosage applied to a desired region of the breast44from the dose of radiation X detected by the exposure control sensor47, and controlling the X-ray tube controller68so as to automatically stop application of radiation X to the breast44when the calculated radiation dosage reaches a predetermined value set as an exposure condition. The X-ray tube controller68and the exposure controller72jointly serve as a radiation source control means.

As shown inFIG. 5, the exposure controller72comprises a current-to-voltage converter74for converting a current signal representative of the dose of radiation X detected by the exposure control sensor47into a voltage signal, an amplifier76for amplifying the voltage signal at a high magnification, an integrator78(radiation dosage calculating means) for integrating the amplified voltage signal with respect to time in order to calculate a voltage signal representing a radiation dosage based on the radiation X. The exposure controller72further comprises an offset voltage calculator80for calculating an offset voltage signal representing an offset value for compensating temperature-dependent and time-dependent changes in the outputs from the circuit components, including the exposure control sensor47, the current-to-voltage converter74, the amplifier76, the integrator78, etc., based on a time-integrated voltage signal output from the integrator78. Finally, the exposure controller72includes a voltage correcting circuit82(radiation dosage correcting means) for correcting the voltage signal output from the integrator78using the offset voltage signal supplied from the offset voltage calculator80.

The offset voltage calculator80is supplied with a resetting signal for resetting the integrator78, and an application start signal for energizing the X-ray tube66to begin applying radiation X to the subject32. During a period of time after the offset voltage calculator80is supplied with the resetting signal and until it is supplied with the application start signal, the offset voltage calculator80functions as a rate-of-change calculating means for calculating a rate of change of the voltage signal output from the integrator78. During this period, the offset voltage calculator80also functions as an offset value calculating means for calculating an offset voltage signal representing an offset value from the calculated rate of change.

The mammographic system12according to the present embodiment is basically constructed as described above. Operations of the mammographic system12shall be described below.

Using the control panel40attached to the mammographic system12, the non-illustrated console, and/or an ID card, etc., the operator enters image capturing information including ID information of the subject32, an image capturing direction, an image capturing region, the tube voltage to be applied to the X-ray tube66, the type of the target54of the X-ray tube66, the type of the filter for dosage adjustment, etc. In the description that follows, it shall be assumed that the operator is capable of setting image capturing information using the control panel40and of confirming the image capturing information displayed on the display panel39.

Having entered the image capturing information, the operator places the mammographic system12into a certain state according to the specified image capturing direction. For example, the breast44may be imaged as a cranio-caudal view (CC) taken from above, a medio-lateral view (ML) taken outwardly from the center of the chest, or a medio-lateral oblique view (MLO) taken from an oblique view. Depending on the selected information of one of these image capturing directions, the operator turns the arm30about the swing shaft28.

Then, the operator places the subject into a specified image capturing state with respect to the mammographic system12. For example, if the breast44of the subject32is to be imaged as a cranio-caudal view (CC), the operator places the subject's breast44onto the image capturing base36, and thereafter lowers the presser plate38to hold the breast44in place between the image capturing base36and the presser plate38, as shown inFIG. 3.

After the breast44has been placed in a desired image capturing state, the subject thickness measuring unit62measures a subject thickness, i.e., the thickness of the breast44, and supplies the measured data to the exposure condition setting unit64.

Using information of the region to be imaged of the subject32, the tube voltage, the type of target54, and the type of filter, which have been entered from the control panel40, the parameters read from the parameter memory60, which include the absorption rate of the region to be imaged where the region to be imaged absorbs the radiation X, the absorption rate at which the presser plate38absorbs the radiation X, the sensitivity of the X-ray detector46, an index depending on the atomic number of the target54, the attenuation characteristics of the radiation X depending on the distance between the target54and the X-ray detector46, and the subject thickness supplied from the subject thickness measuring unit62, the exposure condition setting unit64calculates a tube current to be supplied to the X-ray tube66along with a radiation application time, calculates the dosage of the radiation X that is required to capture a radiation image of the region to be imaged, and sets the calculated values as exposure conditions in the X-ray tube controller68.

For example, assuming the energy of the radiation X to be applied to the X-ray detector46is represented by E, the energy E is expressed by:
E=K·Vn·I·t·S/L2·exp(−μ·d)   (1)
where K represents a characteristic value peculiar to the mammographic system12, V is the tube voltage, n is a tube voltage index, t is the application time of the radiation X, S is the sensitivity of the X-ray detector46, L is the distance between the target54and the X-ray detector46, μ is the absorption rate at which the region to be imaged absorbs the radiation X, and d is the subject thickness. If the energy E (radiation dosage) required in order for the X-ray detector46to be able to detect the radiation X is given highly accurately, then the exposure conditions of the tube current I and the radiation application time t, which ate required to capture the radiation image, can be established according to the above equation (1), using the parameters including the subject thickness d.

After the exposure conditions have been established in the manner described above, the exposure control sensor47is moved in the direction indicated by the arrow inFIG. 3to a position aligned with the mammary gland region of the breast44, for example. Then, a radiation image of the breast44starts being captured. Before the X-ray tube controller68energizes the X-ray tube66to apply the radiation X to the breast44, the X-ray tube controller68supplies resetting signals to the integrator78and to the offset voltage calculator80of the exposure controller72, in order to reset the voltage signal output from the integrator78to 0 V, and also to reset the offset voltage signal calculated by the offset voltage calculator80to 0 V. Then, the X-ray tube controller68is supplied with an application start signal.

After the X-ray tube controller68is supplied with the application start signal, the X-ray tube controller68applies the tube voltage entered from the control panel40to the X-ray tube66, and energizes the X-ray tube66according to the exposure conditions, including the tube current and the radiation application time set by the exposure condition setting unit64. When the tube voltage is applied between the cathode56and the target54of the X-ray tube66, and the tube current set as the exposure condition flows therebetween, the cathode56emits an electron beam “e”. When the emitted electron beam “e” bombards the target54, the target54emits radiation X. The radiation X emitted from the target54passes through the opening58and is applied through the presser plate38to the breast44. The radiation X passes through the breast44and is applied to the X-ray detector46, which is housed in the image capturing base36. Before a radiation image is captured, the entire surface of the X-ray detector46is irradiated with erasing light from the erasing light source50in order to remove unwanted electric charges from the X-ray detector46.

After the radiation X has passed through the breast44, the radiation X carries radiation image information of the breast44. When the radiation X, which carries the radiation image information of the breast44, is applied to the X-ray detector46while a high voltage is applied between the first electrically conductive layer and the second electrically conductive layer, pairs of positive and negative electric charges are generated in the recording photoconductive layer of the X-ray detector46. The negative electric charges are stored in the electric energy storage region that is provided in the interface between the recording photoconductive layer and the charge transport layer. The amount of stored negative electric charge, i.e., the amount of latent image polarity electric charge, is substantially proportional to the dose of radiation X that has passed through the breast44. The positive electric charges generated within the recording photoconductive layer are attracted to the first electrically conductive layer, where they are combined with the negative electric charges of the applied high voltage and hence are eliminated.

The dose of radiation X applied to the breast44is detected by the exposure control sensor47and supplied to the exposure controller72. The exposure controller72calculates a radiation dosage applied to the desired region of the breast44on the basis of the detected dose of radiation X, and supplies the calculated radiation dosage back to the X-ray tube controller68through a feedback loop. When the radiation dosage supplied from the exposure controller72to the X-ray tube controller68reaches the set radiation dosage, which is set as an exposure condition, the X-ray tube controller68outputs an application termination signal in order to stop supplying the tube current to the X-ray tube66. As a result, the radiation image capturing cycle is finished.

Operation of the exposure controller72shall be described in detail below.

Assuming that signals output from the circuit components of the exposure control sensor47and the exposure controller72are ideal and do not depend on temperature, then the time-integrated voltage signal output from the integrator78has a waveform as shown inFIG. 6.

Specifically, if the voltage signal output from the integrator78is not affected by temperature after the integrator78has been supplied with the resetting signal and until the X-ray tube controller68is supplied with the application start signal, then the voltage signal output from the integrator78remains at 0 V. Then, the application start signal is supplied to the X-ray tube controller68to energize the X-ray tube66, which applies the radiation X to the exposure control sensor47. The exposure control sensor47detects the dose of radiation X. The exposure control sensor47supplies a current signal representative of the detected dose of radiation X to the current-to-voltage converter74, which converts the current signal into a voltage signal. The voltage signal output from the current-to-voltage converter74is amplified by the amplifier76, and the amplified voltage signal is supplied to the integrator78. The integrator78integrates the supplied voltage signal with respect to time, and supplies a voltage signal representative of the radiation dosage to the X-ray tube controller68. The X-ray tube controller68compares the voltage signal supplied from the integrator78with a threshold value “a”, which represents a predetermined radiation dosage set as an exposure condition. When the voltage signal agrees with the threshold value “a” at time t1, the X-ray tube controller68outputs an application termination signal to the X-ray tube66, which then stops application of the radiation X.

The above operation is based on the assumption that signals output from the circuit components are ideal. Usually, however, signals which are output from the circuit components of the exposure control sensor47and the exposure controller72tend to vary due to temperature changes and other time-dependent characteristic changes. According to the present embodiment, the voltage signal output from the integrator78is corrected for temperature compensation by the offset voltage calculator80and the voltage correcting circuit82.

During the period of time after the offset voltage calculator80is supplied with the resetting signal and until the offset voltage calculator80is supplied with the application start signal, the offset voltage calculator80calculates a rate of change of the voltage signal output from the integrator78.FIG. 7shows the time-integrated voltage signal output from the integrator78.

Since the exposure control sensor47and the circuit components, including the current-to-voltage converter74, the amplifier76, etc., of the exposure controller72are susceptible to ambient temperature, the integrator78outputs a voltage signal that progressively increases after the integrator78is supplied with the resetting signal and until the X-ray tube controller68is supplied with the application start signal. Assuming that such temperature changes of the exposure controller72, during the short period of time after the resetting signal is supplied and until the application start signal is supplied, can be ignored and the output signal from the integrator78during this period can be approximated by a linear function, then the rate of change a of the voltage signal, which depends on ambient temperature and characteristics of the circuit components, can be calculated on the basis of the time t0when the application start signal is input after the resetting signal has been input, and the voltage signal output from the integrator78at time t0.

The offset voltage calculator80calculates an offset voltage signal Voff as follows, using the calculated rate of change α and the time t from the supply of the resetting signal:
Voff=α·t(2)

Then, when the application start signal is supplied to the X-ray tube controller68to energize the X-ray tube66that applies the radiation X to the exposure control sensor47, the exposure control sensor47detects the dose of radiation X, and then supplies a voltage signal representing the dose of radiation X to the voltage correcting circuit82through the current-to-voltage converter74, the amplifier76, and the integrator78. The voltage correcting circuit82subtracts the offset voltage signal Voff, which is determined by the equation (2), from the voltage signal, and then outputs a temperature compensation voltage signal, which has been corrected using the offset voltage signal Voff, to the X-ray tube controller68.

The X-ray tube controller68then compares the temperature compensation voltage signal from the voltage correcting circuit82with the threshold value “a”. When the temperature compensation voltage signal agrees with the threshold value “a” at time t1, the X-ray tube controller68outputs an application termination signal to the X-ray tube66, which stops application of the radiation X. At this time, the offset voltage signal Voff is expressed as Voff=α·t1.

In the present embodiment, since the offset voltage signal is calculated from the rate of change of the output signal from the integrator78, during a period of time after the resetting signal is supplied and until the application start signal is supplied, and since the output signal from the integrator78is corrected using the offset voltage signal, signals output from the circuit components of the exposure control sensor47, etc., can be temperature-compensated easily with high accuracy. As a result, the radiation dosage of the desired region of the breast44exposed to the radiation X is highly accurately controlled for capturing an appropriate radiation image of the region. The offset voltage signal may be calculated in each radiation image capturing cycle, in order to control an optimum radiation dosage depending on the ambient temperature of the mammographic system12at the time the radiation image is captured.

In the above embodiment, the voltage signal output from the integrator78is corrected by the offset voltage signal in the voltage correcting circuit82, and the corrected voltage signal is supplied to the X-ray tube controller68.FIG. 8shows in block form an exposure controller83according to another embodiment of the present invention. In the exposure controller83, the voltage signal output from the integrator78is not corrected, but rather, is supplied to the X-ray tube controller68, and the offset voltage signal calculated by the offset voltage calculator80is supplied to an exposure condition corrector84(required radiation dosage correcting means). The exposure condition corrector84adds the offset voltage signal to the threshold “a” representing a required radiation dosage set as the exposure condition, thereby setting a new threshold “b” representative of a new required radiation dosage. Then, the X-ray tube controller68compares the voltage signal output from the integrator78with the threshold “b” for controlling the radiation dosage.

After the application termination signal is supplied to the X-ray tube66to terminate the image capturing cycle, the exposure control sensor47is retracted from the position between the X-ray detector46and the erasing light source50. Then, the reading light source48moves in the direction indicated by the arrow along the X-ray detector46while applying the reading light to the X-ray detector46. In the X-ray detector46, pairs of positive and negative electric charges are generated within the reading photoconductive layer, and the positive electric charges are attracted to the negative electric charges (latent image polarity electric charges) stored in the electric energy storage region and move within the charge transport layer. The positive electric charges then combine with the negative electric charges in the electric energy storage region and are eliminated. The negative electric charges generated within the reading photoconductive layer are combined with the negative electric charges supplied to the second photoelectric conductive layer and are eliminated. In this manner, the negative electric charges stored in the X-ray detector46are eliminated by charge combination, whereupon a current is developed within the X-ray detector46due to movement of the electric charges for performing charge combination. The current developed within the X-ray detector46is supplied to the radiation image forming unit70, which produces a radiation image of the breast44based on the supplied current. After the radiation image has been formed, the X-ray detector46is irradiated with erasing light emitted from the erasing light source50in order to remove unwanted electric charges accumulated within the X-ray detector46, and thereby preparing the X-ray detector46for the next radiation image capturing cycle.

In the above embodiments, as described above, exposure conditions are established based on various parameters stored in the parameter memory60, and on information representative of the thickness of the breast44as measured by the subject thickness measuring unit62, wherein a radiation image is captured according to the exposure conditions thus established.

The principles of the present invention are also applicable to an image capturing system in which a “pre-exposure” mode is first performed in order to apply a small prescribed dose of the radiation X to the breast44, and then exposure conditions are established based on the dose of the radiation X that has passed through the breast44. Thereafter, a “main exposure” mode is performed so as to apply the radiation X to the breast44according to the established exposure conditions, for thereby capturing a desired radiation image of the breast44.

Specifically, after the breast44has been positioned on the image capturing base36, a small prescribed dose of the radiation X is applied from the X-ray tube66to the breast44in the “pre-exposure” mode, and then the dose of radiation X that has passed through the breast44is detected by the exposure control sensor47. The dose of radiation X that is detected by the exposure control sensor47is supplied to the exposure condition setting unit64, as indicated by the dotted lines inFIG. 4, and the exposure condition setting unit64establishes the exposure conditions. After the “pre-exposure” mode, a resetting signal is supplied to the integrator78and to the offset voltage calculator80. The offset voltage calculator80calculates a rate of change of the voltage signal output from the integrator78during a period of time after the offset voltage calculator80has been supplied with the resetting signal and until it is supplied with the application start signal for starting the “main exposure” mode. In the “main exposure” mode, an application start signal is supplied to the X-ray tube controller68so as to energize the X-ray tube66, which applies the radiation X to the breast44according to the exposure conditions established during the “pre-exposure” mode. During the “main exposure” mode, the X-ray tube66is controlled based on a radiation dosage, which is corrected based on the rate of change of the voltage signal that has been calculated between the “pre-exposure” mode and the “main exposure” mode.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made to the embodiments without departing from the scope of the invention set forth in the appended claims.