Question

(a) Describe the construction and operation of an indirect digital radiography (DR) image receptor.

(b) Different types of phosphors (composition and structure) can be used in such receptors. What two materials are generally used in these phosphors?

(c) How do these two materials differ in their structure and performance?

(d) What is meant by the modulation transfer function (MTF) of an image receptor?

(e) How do the phosphor structures alluded to in (b) and (c) above affect the MTF of the overall image receptor?

Answer 1

(a)The construction and operation of an indirect digital radiography(DR) image receptor can be described using the features of a digital radiography system.Digital radiography is a type of radiography where x-ray sensitive plates are used to directly capture data where it is used for several purposes.There are many advantages of using this technology such as less radiation and time efficiency.Digital radiography uses a digital image capture device,due to this reason it is effective than x-ray films.

Flat panel detectors are used as receptors here.Indirect digital radiography image receptors are indirect flat panel detectors.combining a-Si detectors with a scintillator in the detectors outer layer,which is made from caesium iodine(CSi) or gadolinium oxysulfide(Gd2O2S),converts x-rays to light.Due to this conversion the a-Si detector is considered an indirect imaging device.The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal.The digital signal is then read out by thin film transistors or fiber coupled charge coupling devices.

(b)A high density line scan solid state detector is composed of a caesium bromide(CsBr)phosphor.The phosphor detector records the x-ray energy during exposure and is scanned by a laser diode to excite the stored energy which is released and read out by a digital image capture array of a charge coupling device.The two materials generally used for these detecctors are phostimulable barium flurobromide doped with europium (BaFBr:Eu) and caesium bromide (CsBr) phosphor.

(c)The main differences between the two materials in their structure and performance are

• The combination of elements are different in both the two structures
• Due to the difference in the structure caesium bromide (CsBr) phosphor is better in the performance

(d)Modulation transfer function of an image receptor is the relation between the image and its quality of an image receptor.The details from an image can be analysed to some particular point with the help of modulation transfer function of an image receptor.

(d)The phosphor structures may affect to the MTF of overall image receptor to a small extend.The modulation transfer function only affect the image and its details.The structure of phosphor does not affect to a greater extend compared to other image receptors which are direct or indirect.