CT Image Quality PDF

CT Image Quality W10 On completion of this section, the student should be able to know the following: Spatial Resolution Contrast Resolution Temporal Resolution CT Number Accuracy (Uniformity an...

CT Image Quality W10 On completion of this section, the student should be able to know the following: Spatial Resolution Contrast Resolution Temporal Resolution CT Number Accuracy (Uniformity and noise) Quality Assurance S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 4/22/2024 1  IMAGE QUALITY IN CT, THE IMAGE QUALITY IS DIRECTLY RELATED TO ITS USEFULNESS IN PROVIDING AN ACCURATE DIAGNOSIS  CT IMAGE QUALITY Spatial Resolution FEATURES USED TO MEASURE CT Contrast Temporal IMAGE QUALITY: Resolution Resolution Noise  SPATIAL RESOLUTION Spatial Resolution—is the ability to distinguish between object or structures that differ in density.  SPATIAL RESOLUTION MEASURED USING TWO METHODS: DIRECTLY, USING A LINE PAIRS PHANTOM. OR BY DATA ANALYSIS KNOWN AS MODULATION TRANSFER FUNCTION (MTF) MTF IS OFTEN USED TO GRAPHICALLY REPRESENT A SYSTEM’S PERFORMANCE.  MEASURE OF SPATIAL RESOLUTION Line pair (lp) : the number of line pairs per unit length is called the spatial frequency, and for CT scanner it is expressed in line pairs per centimeter (lp/cm)..  FACTORS AFFECTING SPATIAL RESOLUTION FIELD OF VIEW AS THE FOV INCREASES , SO A DECREASE IN SPATIAL RESOLUTION PIXEL SIZE THE SMALLER PIXEL SIZE THE HIGHER IN THE SPATIAL RESOLUTION MAGNIFICATION INCREASING MAGNIFICATION WILL DECREASE THE SPATIAL RESOLUTION] PITCH THE HIGHER PITCH WILL DECREASE THE SPATIAL RESOLUTION SLICE THICKNESS THE LARGER SLICE THICKNESS WILL DECREASE THE SPATIAL RESOLUTION  CONTRAST RESOLUTION IT IS THE ABILITY TO DIFFERENTIATE A STRUCTURE THAT VARIES ONLY SLIGHTLY IN DENSITY FROM ITS SURROUNDING. AN EXAMPLE OF AN IMAGING CHALLENGE THAT IS DEPENDENT ON CONTRAST RESOLUTION IS THAT OF A LIVER LESION SURROUNDED BY HEALTHY LIVER TISSUE. CT IS SUPERIOR TO ALL OTHER CLINICAL MODALITIES IN ITS CONTRAST RESOLUTION. FOR COMPARISON, IN SCREEN-FILM RADIOGRAPHY, THE OBJECT MUST HAVE AT LEAST A 5-10% DIFFERENCE IN CONTRAST FROM ITS BACKGROUND TO BE DISCERNIBLE ON THE IMAGE. ON CT IMAGES, OBJECTS WITH A 0.25-0.5% CONTRAST VARIATION CAN BE DISTINGUISHED  TEMPORAL RESOLUTION TEMPORAL RESOLUTION REFERS OF A SYSTEM TO HOW RAPIDLY DATA ARE ACQUIRED. CONTROLLED BY: GANTRY ROTATION SPEED. NUMBER OF DETECTOR CHANNELS IN THE SYSTEM. HIGH TEMPORAL RESOLUTION IS OF PARTICULAR IMPORTANCE WHEN IMAGING MOVING STRUCTURES (E.G., HEART) AND FOR STUDIES DEPENDENT ON THE DYNAMIC FLOW O (E.G., CT ANGIOGRAPHY).  ACR CT ACCREDITATION PHANTOM ACR (AMERICAN COLLEGE OF RADIOLOGY) CT PHANTOM CONSISTS OF FOUR MODULES THAT CAN BE USED TO EXAMINE A BROAD RANGE OF SCANNER PARAMETERS. Romans L, Computed Tomography for Technologists: a comprehensive text, 2011, pg 72.  1- CONTRAST RESOLUTION QA 2- SLICE THICKNESS AND LASER LIGHT ACCURACY QA 3- CT NUMBER QA 4- NOISE AND UNIFORMITY QA 5- LINEARITY QA Course Instructors Dr. Saddig Jastaniah & Dr. Jamaan Al-Ghamdi 4/22/2024 12  CONTRAST RESOLUTION QA ASSESSMENT OF LOW-CONTRAST RESOLUTION IS PERFORMED THROUGH THE USE OF PHANTOM CONTAINING OBJECTS OF VARIABLE SIZES WITH A SMALL DIFFERENCE IN DENSITY FROM THEIR SURROUNDINGS. FREQUENCY - MONTHLY BASIS.  SLICE THICKNESS AND LASER LIGHT ACCURACY QA SLICE THICKNESS QA IS MEASURED USING A PHANTOM THAT INCLUDES RAMP OR STEP-WEDGE OF KNOWN MEASUREMENTS. LASER LIGHT FROM THE GANTRY CAN BE EVALUATED BY SPECIFIC PHANTOM PROVIDED BY THE MANUFACTURER. FREQUENCY: BOTH SEMIANNUALLY.  CT NUMBER QA (WATER & AIR) THIS QA TESTS THE AVERAGE CT NUMBER OF WATER. WATER SHOULD BE 0 ± 3 CT NUMBERS. THE AVERAGE CT NUMBER OF AIR IS -1000 ± 5 CT NUMBERS. FREQUENCY: DAILY BASIS.  NOISE AND UNIFORMITY QA BOTH CAN BE MEASURED USING A WATER PHANTOM. NOISE - BY OBTAINING THE STANDARD DEVIATION OF THE CT NUMBERS WITHIN A SINGLE ROI. UNIFORMITY - BY PLACING SEVERAL ROIS WITHIN THE PHANTOM. FREQUENCY: BOTH WEEKLY.  Uniformity QA Romans L, Computed Tomography for Technologists: a comprehensive text, 2011, pg 73.  LINEARITY QA LINEARITY CAN BE MEASURED USING A PHANTOM CONTAINING A VARIETY OF OBJECTS WITH KNOWN DENSITIES. MEASURED VALUES CAN BE COMPARED WITH STANDARD KNOWN VALUES. FREQUENCY: SEMIANNUALLY.  Linearity QA Romans L, Computed Tomography for Technologists: a comprehensive text, 2011, pg 73.