Международного межуниверситетского семинара по диагностической и терапевтической радиологии
Минск, 20-21 октября 2003 года
Image Quality: Basis of successful therapy.
Dr. Med. F. Dhaenens.
Agfa, Mortsel, Belgium.
(Радиология в медицинской диагностике [современные технологии] 2003: 52-53)
Image quality is the basis for diagnostic quality in all modalities, which
in turn should give a better therapeutic management and a better outcome for
Several extensive studies show that subtle signs in radiological images can be overlooked, as is often the case with small lung nodules. Up to 30% of lung nodules in non-suspected patients, are missed, because lack of attention, low image quality or failing conspicuity. Other studies reveal that a variation of +/- 50% of the optical density in the lungfields can lead to 20-30% of missed lesions, simply because they are not any longer detectable for the human eye. Radiology is the combination of a detection task, based upon prior information about the patient, and experienced knowledge. Happily radiologists rarely depend solely on single signs.
Exposed with the optimal dose, x-ray film has an excellent quality, but detoriates rapidly with under- and overexposure due to the limited exposure window and the non-linearity of the film characteristics which are the cause of irreversible loss of data, many time more worse than high degree digital compression.
Repeat rates with film/screen systems are higher than 10% in most of the radiology departments, while repeat rates are less than 1% with digital systems. This means less labor, less materials, less dose, but also an improvement in outcome of the patient.
Image quality is physically determined by 3 parameters: Signal/ Noise ratio, resolution (in Lp/mm) and contrast. This process is well known for filmbased imaging, but is relatively new for digital imaging. The relations between these physical parameters have a significant meaning for radiology: the link between resolution and contrast is called “Modulation Transfer Function” (MTF) and characterizes film systems and digital systems completely. MTF determines how good small details will be observed and displayed by imaging systems.
Most of the digital detectors perform better for signal/noise and contrast than film/screen systems do, but less good for spatial resolution.
This teaches us that digital systems combined with advanced contrast image
processing will provide radiology with a stable and superior image quality compared
to film. The image processing is based upon contrast processing (like in CT
and MRI) and compensates for the limited resolution of digital systems.
Digital displays (like CRT monitors or flat displays) have limited characteristics (light power and dynamic range) compared to film, but this can easily be solved by the digital image processing.
Digital systems are available in many forms and brands, which can be divided in 2 classes: CR or Computed Radiography, an economical solution, which replaces the film-screen systems by cassettes with a reusable stimulable phosphor screen inside and the DR or flat panel detectors, using chip technology with scintillators. Today the ideal system does not yet exist, but in general all systems offer a reasonable to excellent image quality. These digital detectors combined with an appropriate image processing working on the contrast generate high quality and day after day image stability. Image processing is a critical part that determines how good the workflow of the radiologist will become. Modern multiscale or multifrequency enhancing processing like MUSICA? of AGFA optimizes the image quality and push repeat rates below 1%.
Diagnostic mammography and mammographic screening are examples where image quality of film and digital systems are improving drastically the outcomes of the patient survival, supposed that the treatment and the follow-up is appropriate. Due to the high thruput of cases in screening, the workpressure on radiologists is high and the help of contrast enhancing techniques and CAD (Computer Aided Detection) is welcome. The same is happening with lung cancer screening with plain x-ray images where CAD for nodules will play a role.
The use of CR systems in radiotherapy for simulation and verification is another example where the improvement of image quality by digital techniques is advancing the treatment quality for the patient.
Different from film, the process of creating image quality with digital systems is using different paths, but essential the same parameters like signal, resolution and contrast are used. Therefore the understanding of the “digital” image creating process as well as image processing, is essential in order to obtain the best results and to take the right decisions to walk into this new area.