Seeram Chapter 9: Image Manipulation in CT

Image Manipulation Defined “Those techniques (operations) or processes which modify an image or group of images to enhance the visibility of useful information while suppressing ‘noise’ or non-useful information” No additional or new information produced Content of interest emphasized Overall information content reduced

Windowing Manipulation of image gray scale using image’s CT numbers

Windowing Manipulation allows customization of visibility soft tissues brain dense structures bone

Window Width & Level WL WW Window width Window level range of CT #’s imaged determines maximum # of gray shades which could be displayed on monitor Window level center or midpoint of CT # range WW WL

Window Width & Level Pixels outside of window displayed as Black or White WW WL

Window Width & Level 3000 >200 151-200 2000 101-150 51-100 1-50 1000 (-49)-0 (-99)-(-50) Window: 400 Level: 0 (-149)-(-100) (-199)-(-150) <(-199) -1000

Small Window Width Short gray scale 1000 3000 Short gray scale Small block of CT #’s assigned gray levels Small transition zone of white to black Window: 400 Level: 0 -1000 -1000

Small Window Width 1000 3000 Used to display soft tissues within structures containing different tissues of similar densities Level centered near average CT # of organ of interest Window: 400 Level: 0 -1000 -1000

Large Window Width Long gray scale 1000 Long gray scale Large block of CT #’s assigned gray levels Large transition zone of white to black Window: 2000 Level: 0 -1000

Large Window Width Used where large latitude required 1000 Used where large latitude required Used to simultaneously display tissues of greatly differing attenuation Window: 2000 Level: 0 -1000

Window Example 100 WL =0 WW = 200 All pixels with CT #’s > 0 +(200/2) = 100: White 200 All pixels with CT #’s < 0 -(200/2) = -100: Black -100

Another Window Example 140 WL = 40 WW = 200 All pixels with CT #’s > 40 + (200/2) = 140: White 200 40 All pixels with CT #’s < 40 - (200/2) = -60: Black -60

Still Another Window Example 200 WL = 0 WW = 400 All pixels with CT #’s > 0 + (400/2) = 200: White 400 All pixels with CT #’s < 0 - (400/2) = -200: Black -200

Larger Window Means Obscuring Small Differences in Tissue Attenuation WW=200 WW=400 200 100 Range 20 - 40 40 - 80 20 40 One gray shade encompasses larger range of CT #’s -100 -200

Window Width & Contrast As WW increases contrast decreases latitude (range of CT #’s imaged) increases As WW decreases contrast increases latitude decreases Clinical goal: Largest available contrast at the latitude required by study

Window Width & Image Contrast Large window width different structures more likely to have same gray shade Narrow window width Gray shade differences more likely visible between structures Very narrow window width Small differences in attenuation seen as black & white

Preset Window & Level Available for all commercial CT initial WW and WL pre-sets for specific study types Can be overridden by operator

Specialized CT Image Manipulation Programs Region of interest (ROI) analysis average CT # standard deviation

Specialized CT Image Manipulation Programs Image annotation grids arrows notes Bad Thingee

Specialized CT Image Manipulation Programs Histogram analysis graph of CT # frequency # of pixels with given range of CT#’s can apply to ROI entire image

Specialized CT Image Manipulation Programs Multiplanar reconstruction 3D reconstruction Quantitative CT Osteo CT / bone densitometry Xenon CT Radiation therapy treatment planning using CT data

Multiplanar Reconstruction Creates non-axial images from stack of contiguous transverse axial scans stack contains 3 dimensional CT data pixels new cut identified & selected from each axial image without scanning

Cuts coronal sagittal paraxial oblique

Coronal Reconstruction Example Stack of Axial Slices Reconstructed Slice

Coronal Reconstruction Example Stack of Axial Slices Reconstructed Slice

Sagittal Reconstruction Example Stack of Axial Slices Reconstructed Slice

Oblique Reconstruction Example Stack of Axial Slices Reconstructed Slice

Reformatting Advantages enables visualization of specific structures relative to surrounding structures aids in determining / localizing true extent of lesions fractures bone fragments foreign bodies

Reformatting Disadvantages Image quality can be poorer than axial images if plane thickness > pixel size affects blurring less problem in spiral scanning

Reformatting Disadvantages More prone to motion / breathing artifacts reformatted image taken from many slices reformatted image represents longer time interval than single slice

Quantitative CT: Measurement of Bone Mineral Density (BMD) Competes with DEXA

Quantitative CT: Measurement of Bone Mineral Density (BMD) Scout image used to define midvertebral plane Axial images contains both patient & reference phantom Reference phantom used to account for slight CT # changes over time phantom has water & bone equivalent parts

Quantitative CT: Measurement of Bone Mineral Density (BMD) Software automatically defines region of interest Mean CT #’s calculated Bone Mineral Density calculated BMD compared to “standard” values previous studies for this patient

Xenon CT for Regional Cerebral Blood Flow Imaging Patient inhales xenon gas serves as a contrast agent Xenon gas inert element high atomic # provides good CT contrast high fat solubility crosses blood - brain barrier

Xenon CT Applications Brain metabolism Potential applications regional perfusion in cerebrovascular disease Potential applications dementia sleep disorders migraines epilepsy

CT in Radiation Therapy Treatment Planning Conventional CT study performed patient positioned exactly as for therapy CT data communicated to treatment planning computer

CT in Radiation Therapy Treatment Planning CT data used to develop depth dose data Isodose data superimposed on CT image

Other CT Software Features Multiple window settings within a slice Image magnification Measurement of distances / angles Highlighting of selected pixel values in image Cine viewing of image stack

CT Applications Split image Dental Angiography one thicker slice reconstructed into two thinner slices Dental Angiography Plastic Surgery Reconstruction Multimodality Image Fusion Overlaying of images of various modalities One anatomical (CT), one functional (Nuc med)