CT Image Quality and Display PDF

CT Image Quality and Display CT Image Quality there are 5 methods for measuring CT image quality 1. spacial resolution 2. Contrast resolution 3. Noise 4. Linearity 5. Uniformity Spacial Resolut...

CT Image Quality and Display CT Image Quality there are 5 methods for measuring CT image quality 1. spacial resolution 2. Contrast resolution 3. Noise 4. Linearity 5. Uniformity Spacial Resolution -If one images a regular structure that has a sharp interface the image at the interface will be somewhat blurred -The degree of blurring is a measure of the spatial resolution of the system -The smaller the pixel the better the spacial resolution -spacial resolution for all imaging modalities is determined by the pixel size -A low spacial frequency represents large objects -A high frequency respresents small objects -Is a measure of the scanners ability to seperate 2 objects that are located adjacent to one another such as small calcifications -directly affected by the detector element size -thinner slice thickness also allows for higher spatial resolution -voxel size also effects this -expressed as lp/cm (line pairs per cm) CT Image Quality and Display 1  Contrast Resolution -The ability to distinguish one soft tissue from another without regard to size or shape is called contrast resolution -This is an area that multislice helical CT excels -The attenuation coefficient is a function fo x-ray energy and the atomic number of the tissue(density) -In CT absorbtion of x-rays bt the patient is also determined by the mass density of the body part (just like x-ray bone is going to absorb more than tissue) -Amplified contrast scale allows CT to better resolve adjacent structures that are similar in composition using WW and WL -The ability to image low-contrast objects with CT is limited by the size and uniformity of the object and the noise of the system Contrast resolution and the HU (Hounsfield Unit) -CT is amazing for differentiating objects with very similar densities -this is because of thinner image slices with no superior or inferior objects -Solid state detectors are better than film Noise CT Image Quality and Display 2  -If water is imaged each pixel should have a value of 0 because the HU number is 0 -This never occurs because of contrast resolution therefore CT number may adverage 0 but every individual pixel is not going to equal 0 -The variation in CT numbers below or above the 0 value is the noise of the system -If all pixel values were equal then the noise would be zero -Noise depends on kVp, filtration, pixel size, slice thickness, detector efficiency, Patient radiation dose -A large variation of pixel values respresents high image noise -The resolution of low contrast objects is limited by the noise of a CT imaging system -Noise appears on the image as graininess -Low noise is smooth to the eye, high noise is spotty or blotchy High noise Low noise Linearity -refers to the relationship of CT numbers to the linear attenuation of the object being imaged Uniformity CT Image Quality and Display 3  -When a uniform object such as water is imaged each pixel should have the same value because each pixel represents the same object (water) -if the CT system is properly adjusted the value should be 0 -The CT value for water may drift from day to day or even hour to hour CT Image Manipulation and Display Image display localizer Scan (Scout) -Most CT scans begin with localizer images -Similar to images taken in conventional x-ray -created while the tube is stationary -In all routine scans a localizer image is needed (usually AP and lateral) -Allows the CT tech to see where all anatomy is and to ensure it is in frame Image Enlargement -Enlarges pixels therby decreasing resolution -Rotation or reversal of images CT Image Quality and Display 4  Image Filtering (algorithms) -Smoothing by reducing image noise and good soft tissue discrimination -Edge enhancement by lowering noise to help detect smaller masses or bone fractures Window width and Window Level Affect -Different window width and levels for different body parts to help with contrast in similar density body parts Image Manipulation -Called image reconstruction -Take raw data from the CT scan and manipulate or reconstruct into different images Includes: 1. Filters (algorithms) 2. DFOV (Display Field of view) 3. MPR (multiplanar reconstruction) 4. MIP (Maxium intensity projection) CT Image Quality and Display 5  5. 3D imaging (Volume and surface rendering) Multiplanar Reconstruction MPR -Computer program that allows images to go from axial or transverse to coronal, saggital and oblique Saggital view (right to left side) CT Image Quality and Display 6  Coronal view (anterior to posterior) Oblique or irregular view (on any angle) -Multislice helical CT excels in 3D MPR (multiplanar reformation) -Transverse images can be stacked to form a 3D image using -Maximum intensity projection (MIP) -Shaded Surface Display (SSD) -Shaded Volume Display (SVD Maximum Intensity projection (MIP) CT Image Quality and Display 7  -Reconstructs the image using the highest value pixels along any line and only exhibiting those pixels -Widely used in angiography -only appox 10% of the three dimensional data points are used -This can create a very high contrast image of contrast filled vessels -Small vessels that pass obliquely through a voxel may not be imaged because of the partail volume adveraging Shaded Surface Display (SSD) -The range displayed appears as an organ surface that is determined by operator selector values -Surface boundaries can be made distinctive and can provide an image that appears very 3D CT Image Quality and Display 8  -allows for addition and deletion of body parts Shaded Volume Display (SVD) -Produces a 3D image that has better image quality and provides more information compared to SSD -Uses the entire data set -Two stages 1. Preprocessing- assigns different brightness levels or colours to tissue types, a partial transparency is also assigned (Fat, soft tissue) 2. Rendering-Involves image projection to form simulated 3D image -Offers the advantage of seeing through surfaces to allow the viewer to see interior and exterior of the imaged structures CT Image Quality and Display 9