Image acquisition

Radiological images today are acquired utilizing a wide spectrum of different techniques, ranging from analogue via various acquisition modalities to direct digital registration. The technique is usually dependent on the radiological modality in question.

Conventional radiology techniques usually acquire an analogue image on photographic film where the density varies continuously without any discrete steps. The image is normally formed through direct or indirect radiation on the emulsion layer of the film by X-rays emanating from an X-ray tube and by light emitted by the intensifying screens (see Chapter 4). This category also contains techniques such as scanograms and ultrasonography where analogue image information is amplified electronically and transmitted as a video signal to image screens and photographic film. The video signal is the means by which analogue information is transmitted electronically.

The images created on image plates, during digital fluoroscopy, digital subtraction angiography, in certain ultrasonographic equipment, and in gamma cameras are initially registered in digital form. These data can subsequently be transformed and presented either in analogue or digital form on image screen or photographic film.

Finally, we have the group of modalities which comprised the breakthrough for digital techniques in radiology, namely computed tomography (CT), and later Single Photon Emission Computerised Tomography (SPECT), Positron Emission Tomography (PET), and Magnetic Resonance Imaging (MRl). In these modalities transmitted or emitted electromagnetic photons not primarily depicting anatomy are registered and the image is calculated by computer from the photon information. They are thus producing calculated digital images and the density of each pixel has been obtained as the solution of a series of equations.

Analogue/digital transformation

Image content, transmitted by electronic or optical means within radiological equipment, a radiological department, or between different departments is sent in analogue form via an electric current or by an optical signal varying in intensity in a similar manner to that shown in the curve in Fig. 1 a. The image can also be transmitted as binary digital signals demonstrated in Fig. 1 d.

Acquisition of radiological information as well as presentation of images on a monitor are thus sometimes digital and sometimes analogue. This means that there is a need for units that convert electronic image information from analogue to digital and vice versa. Such units are called A/D and DIA converters.

Digital image plates

The radiological image on a photographic film is produced through illumination with light from intensifying screens and to a lesser degree as a direct effect of X-ray quanta. On digital image plates the information is stored as excited electrons in phosphorous plates containing complex inorganic substances.

Thus, the radiological information, after passage of the radiation through the object/body, is temporarily stored on the image plate in inverse proportion to the absorbed radiation dose. In order to read this information a laser beam is used to release the trapped energy stored in the image plate as emitted light. The laser beam sweeps over the image plate line by line until the whole surface has been read. After the laser stimulation the stored information is transformed from emitted light to electronic signals using a photo multiplier. The image plate can be reused many thousands of times.

The electronic signal is amplified, digitized, and transferred to an image processor. This unit has a twofold function. Firstly, it calculates the mean darkness of the image and the total range between the light and the dark parts of the image. Secondly, it uses this information to calculate an optimal image that is transmitted to a laser printer which produces the image on transparent film.

The digital information can also be transmitted to a Picture Archiving and Communications System (PACS), which used in combination with high resolution monitors for diagnostic work and conferences can eliminate the use of photographic film.

The use of digital images from image plates has a number of advantages:

1. There is increased sensitivity to X-ray quanta which can be utilised to increase the image quality and/or reduce the radiation dose.
2. Erroneous exposures are greatly reduced as nearly all exposures can be salvaged by the image processing.
3. The digital image can be transmitted, manipulated, and presented in many different ways.

Calculation of a tomographic image

CT and other modalities where the image is calculated from emitted or transmitted electromagnetic information, use tomographic sections. This means that the calculated images represent parallel planes through the anatomy in question. Such a section is defined as the plane in which the detectors of the CT, SPECT, or PET units are located. In MRI, the image reconstruction can be made in any selected plane (2D) or volume (3D).

The calculations involved are complex and time-consuming even for powerful computers. As a simplified description in the case of CT each
detector reading can be said to represent the absorption along the line of volume elements through which the X-ray beam has passed.

This will give an equation where the absorption in each single volume element is the unknown parameter that represents the darkness of the corresponding pixel. As a large number of detector readings is made, a huge equation system is obtained, for example in a 512 x 512 matrix there are 262,144 unknown parameters.

These equations are solved with different, more or less approximate mathematical techniques. The result will be a density value for each pixel of the tomographic image.

Spatial and density resolution

The most important aspect of image quality is resolution. The number of line pairs per mm that can be resolved by the eye under defined circumstances is often used. However, this definition is only valid for analogue images. Digital images cannot of course resolve details that are smaller than one pixel. This type of resolution is called spatial resolution, compare Figs. 2 a, b.

If the spatial resolution of a conventional radiological film is determined it is comparable to a digital image with a resolution as high as 4096 x 4096 pixels. Under certain circumstances even higher spatial resolution is used, e.g. in mammography.

In order to assess the spatial resolution the contrast resolution has to be maximal. This means that the line pairs and the background have to be black and white. If the contrast resolution is less the spatial resolution will be reduced. The contrast resolution in a digital image depends upon the number of possible shades of darkness and is often better than that of an analogue image.

Tatsuo Kumazaki and Hans Ringertz