Image Quality PDF

IMAGE QUALITY Mohd Hafizi Mahmud Centre for Medical Imaging Studies Faculty of Health Sciences Universiti Teknologi MARA Puncak Alam Campus  High Contrast Spatial Resolution  Describes the scanner’s ability to resolve closel...

IMAGE QUALITY Mohd Hafizi Mahmud Centre for Medical Imaging Studies Faculty of Health Sciences Universiti Teknologi MARA Puncak Alam Campus  High Contrast Spatial Resolution  Describes the scanner’s ability to resolve closely placed objects that are significantly different from their background In Plane Spatial Resolution  Line pairs per centimeter (lp/cm) or line pairs per millimeter (lp/mm).  Factors Affecting Resolution  X-ray focal spot size and shape, detector cell size, scanner geometry, sampling frequency – determined by CT manufacturers.  Reconstruction algorithm (strongly influenced)  Reconstruction field of view (FOV).  Low Contrast Resolution  sensitivity of the system  0.25% to 0.5% (CT) vs 10% (radiograph)  Can be measured with phantoms that contain low- contrast objects of different sizes.  Low-contrast performance or low-contrast detectability (LCD) of the scanner is defined as the smallest object that can be visualized.  Factors That Affect Low-Contrast Detectability  Noise  Slice thickness  Temporal Resolution  Ability of the CT system to freeze motions of the scanned object.  Linearity  The relationship of CT numbers to the linear attenuation coefficients of the object to be imaged  CT number is related to the attenuation coefficient of the object by the following equation:  Uniformity  For a uniform phantom, the CT number measurement should not change with the location of the selected regions of interest (ROI) or with the phantom position relative to the isocenter of the scanner.  Considering the effects of beam hardening, scatter, stability of the CT system, and many other factors, the CT number uniformity can only be maintained within a reasonable range (typically 2 HU).  Noise Factors influencing noise : Quantum noise  Determined by x-ray flux or the number of detected x-ray photons. It is influenced by the scanning techniques(e.g., x-ray tube voltage, tube current, slice thickness, scan speed, and helical pitch), scanner efficiency (e.g., detector quantum efficiency, detector geometrical efficiency, amber- penumbra ratio), and patient (e.g., patient size, amount of bones and soft tissues in the scanning plane). Inherent physical limitations of the system  Including electronic noise in the detector photodiode, electronic noise in the data acquisition system (DAS), scattered radiation etc. Reconstruction parameters  High-resolution reconstruction kernel produces an increased noise level.