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Radiological Protection of Patients |
There is no objective definition of image quality; it is more a matter of the observer’s subjective judgement. In nuclear medicine, the basis of image quality is the ability of the imaging device to detect differences in the uptake of a radiopharmaceutical in a lesion and its surroundings. Hence, an image of high quality is one that can reproduce this contrast in order to secure a correct diagnosis.
However, several factors will degrade the image quality, some of which are due to inherent properties of the imaging device such as spatial resolution, energy resolution, non-uniformity, or distortions. Other degrading factors are dependent on the patient and organ localization. A large patient will increase the influence of scattered photons. An organ deep in the body will be overlapped by other tissues, which will increase the background registrations. Patient and organ movements will also degrade the image quality.
Finally, some important factors are due to the operation of the imaging device and can be optimized by the user. These include spatial resolution by keeping the distance between the detector and the patient as short as possible and noise reduction by selecting an optimum examination time and matrix size. Scattered radiation can be reduced by a proper setting of the pulse height analyser.
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Left: A simulated true activity distribution of a radiopharmaceutical. Right: The same image degraded by attenuation of photons and addition of scattered photons and noise. (Courtesy: M. Ljungberg, Lund, Sweden) |
Optimization of protection in nuclear medicine is to ensure that the exposure of patients be the minimum required to achieve the intended diagnostic objective (BSS) which requires to reach a balance between the patient exposure - and hence the radiation risk - and the diagnostic accuracy. The relation between activity of the radiopharmaceutical and diagnostic accuracy is highly dependent on the type of examination. It is also important to know whether the diagnosis is based on quantitative information or on visual evaluation. Both for a simple uptake measurement and in connection with imaging, the amount of activity needed will depend on the type of equipment used, the body constitution of the individual patient, the patient’s metabolic characteristics and clinical condition.
The administration of amounts substantially larger than the optimum in order to marginally improve the quality of the results obtained should be discouraged.
Equipment must be operated within the limits and conditions established in the technical specifications and in the license requirements, ensuring that it will operate satisfactorily at all times, in terms of both the tasks to be accomplished and radiation safety.
The data acquisition conditions should be chosen such that the image quality is optimum. The choice of collimator, energy window, matrix size, acquisition time, angulations of detector, SPECT or PET parameters, and zoom factor must be such as to obtain optimum quality image. For dynamic studies, the number of frames, time interval and other parameters should be chosen to obtain optimum quality of image sequence.
The patient should be fully informed about the examination. Patient factors such as age, disease, and size should be considered in the optimization of the examination.
Optimization, in general, will also mean optimization of the imaging technique in order to achieve best possible image quality with the available equipment. Image quality is dependent on technical factors and on patient related factors such as age, size and disease. Technical factors include the properties of the equipment used, acquisition protocol, image processing, image noise, spatial resolution, and scattered radiation. To ensure the best possible use of available resources, various technical factors involved in a nuclear medicine investigation should be optimized for every type of examination.
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Among many other factors, type of collimator, the distance between the camera collimator and the patient is one that affects the image quality substantially. The left image is acquired with shortest possible distance and the right image is acquired with a distance of 15 cm. |
The major source of noise is the random distribution of photons per picture element detected by the gamma camera. The noise level is generally very high compared to other imaging modalities. A high noise level will obscure the contrast and reduce the image quality, resulting in less accurate diagnosis. In a grayscale image, the human eye cannot detect contrasts <10% even in the absence of noise.
To reduce noise, increase either the examination time or the administered activity. Increasing the administered activity means increasing the patient exposure. The total number of registrations must, however, be a compromise between examination time and the possibility for the patient to keep still during the examination.
Another way to lower the noise level is by increasing the pixel size. Changing the matrix from 256x256 to 128x128 pixels, for example, reduces the noise level by a factor of two. However, the pixel size should not be too large so as not to influence the spatial resolution of the image.
Finally, image processing such as background subtraction and digital filtering can be used to reduce the noise level and increase the contrast.
A high noise level will reduce the ability |
Reducing the total number of registrations in the image |
Yes and no. Here are some typical questions and answers for illustration.
Yes. A quality control (QC) programme is aimed at maintaining high performance of equipment regardless of its age. It starts with the initial acceptance test, which consists of a QC-programme to assess whether the equipment fulfils the specifications or not.
Routine QC measurements should be made at regular intervals and also after a major change of components, updating by the manufacturer, or repairs. Since it is essential to maintain long term overall stability of performance, these measurements must be carefuly specified, performed, recorded, and evaluated.
Upper row: False positive bone scan caused by contamination of the gamma camera collimator.
Lower row: False positive myocardial scan due to a defective collimator.
