B ASIC P HYSICS OF D IGITAL R ADIOGRAPHY By : Maisa Alhassoun

I MAGING WITH F ILM The greatest challenge of film radiography is to get the range of exposure to fit into the dynamic range of the film. If the exposure falls outside the latitude, there will be little or no image contrast formed.

I MAGING WITH F ILM There are two conditions that contribute to receptor exposure outside the film latitude: Error in setting the correct exposure factors. Some regions of the body have wide dynamic of exposure that exceeds the latitude of the film. Using a wider latitude film can correct this but results in lower image contrast.

T HE A DVANTAGE OF A W IDE D YNAMIC R ANGE Here we can see the advantages of a digital receptor that has a wide dynamic range. Even when there is a wide range of exposure coming from the body (wide histogram) and exposure at different levels (exposure errors) they still fit within the dynamic range.

D IGITAL I MAGE C ONTRAST In a digital image, contrast is represented by different pixel values. A typical digital receptor will have a linear relationship between exposure and resulting pixel value.

D IGITAL I MAGE C ONTRAST We have seen that this relationship extends over a relatively wide range of exposures. This is contrasted with the non-linear relationship seen with film.

O PTIMUM E XPOSURE IN D IGITAL R ADIOGRAPHY The wide dynamic range and linear response of digital receptors is like a double edged sword. The advantage is a wide range of exposures and exposure errors will still produce an image with good contrast.

O PTIMUM E XPOSURE IN D IGITAL R ADIOGRAPHY So, what is the problem? There are two that we can see here. Even though good contrast with low exposure is obtained. Due to the low exposure, we have high noise.

O PTIMUM E XPOSURE IN D IGITAL R ADIOGRAPHY With film the image would be light. The other problem is that exceptionally good images are obtained with high exposure ( very low noise). With film the image would be dark or over exposed

O PTIMUM E XPOSURE IN D IGITAL R ADIOGRAPHY In general, the challenge is to make sure the exposure factors are set for optimum image quality.

M ONITORING E XPOSURE L EVELS One of the challenges is to know when the digital image is properly exposed since it is unlike film where under and over exposure is obvious. Each manufacturer of digital receptors will provide an approach to calculate the image exposure information.

M ONITORING E XPOSURE L EVELS Some will display the “S” number. This displayed value generally indicates the speed of the receptor that would match the actual exposure.

M ONITORING E XPOSURE L EVELS A low exposure would result in a high calculated S number (like S=1000) and a high exposure would produce a low number (like s=50). This is opposite of screen-film speed.

M ONITORING E XPOSURE L EVELS The operator should determine the appropriate range of values for optimum exposure and monitor the values. The method varies by manufacturer and clinical procedure.

ACR E XPOSURE F ACTOR C HART Fuji (S Number) Agfa (LGM value Kodak (Exposure Index) Detector Exposure Estimate mR Indication & Action >1000<1.45<1250<0.20Repeat Underexposed QC exception Underexposed QC review Acceptable < > > >4.0 Overexposed Repeat

D IGITAL E XPOSURE Proper exposureOver exposure

D IGITAL R ADIOGRAPHY I MAGE Q UALITY Like all x-ray images, there are 5 specific quality characteristics. Spatial detail Detail Contrast Noise Artifacts We will now see how 3 of these, contrast, detail and noise are effected by the operation of digital radiography.

D IGITAL R ADIOGRAPH C ONTRAST Contrast sensitivity of digital procedures and image contrast depend upon several factors. Two are: X-ray beam spectrum Scattered radiation Similar to film.

D IGITAL R ADIOGRAPH C ONTRAST What is different is the ability to adjust and optimize contrast after the image is recorded. This is done through the digital processing of the image and then the adjustment of the window when the image is being viewed.

D IGITAL R ADIOGRAPHIC D ETAIL Visibility of detail is reduced and limited by the blurring that occurs at different stages of the imaging process. Some are common to both film and digital radiography.

C OMMON S OURCES OF B LURRING Common sources of blurring; The focal spot (depends upon size and object location.) Motion if present The receptor (generally due to light spreading within the fluorescent or phosphor screen)

D IGITAL S OURCES OF B LURRING Additional blurring is the result of dividing the image into pixels. The size of a pixel (amount of blurring) is the ratio of the image size (image size relative to the anatomy) and the matrix size.

D IGITAL S OURCES OF B LURRING Pixel size is another factor that must be considered because it limits the detail of the image x 1280 on a 18cm x 24 cm image would be sharper than on a 35 cm x 43 cm image. Generally, digital images lack the spatial detail of film images due to the digital processing.

N OISE IN D IGITAL R ADIOGRAPHS The most predominate source of noise in digital imaging is quantum noise associated with the random distribution of the x- ray photons received by the image receptor.

N OISE IN D IGITAL R ADIOGRAPHS The level of noise depends upon the amount of receptor exposure used to produce the image. With digital imaging, it can be adjusted over a wide range due to the wide dynamic range.

N OISE IN D IGITAL R ADIOGRAPHS Noise is controlled by using the appropriate exposure factors.

D IGITAL I MAGE N OISE UnderexposedProper exposure