Three Dimensional Computed Tomography: Basic Concepts Chapter 17 Seeram

Why 3-D? Can be used to aid in the study of AIDS, Huntington’s disease, and schizophrenia –This is done by using the 3D model as a map to determine areas most affected by disease processes Models can be used to more accurately show tumor shape and size for radiation therapy planning 3D imaging is beginning to gain acceptance as a tool for virtual colonoscopy by allowing the viewer to “fly through” the colon –The downside is that no tissue samples can be obtained during this procedure

Why 3D? 3D imaging has been used to study Egyptian mummies without destroying the plaster or bandages 3D imaging aids in the diagnosis of vascular pathology 3D imaging can be used to plan surgery and is often used during surgical procedures –Real time 3D information shows the surgeon where the cuts are being made in relation to critical anatomy and pathology

History of 3D Greenleaf et al produced a motion display of the ventricles using biplane angiography –Greenleaf JF, Tu TS, Wood EH (1970) Computer-generated three-dimensional oscilloscopic images and associated techniques for display and study of the spatial distribution of pulmonary blood flow. IEEE Trans Nucl Sci NS-17: Using the information gained from Greenleaf et al, it was clear that contiguous CT images could be stacked in a fashion that would create a 3D image

History of 3D Soon software and hardware became available to ease the production of 3D images –Along with the hardware and software came algorithms for 3D imaging By the 1980’s many CT scanners offered 3D software as an optional package

Early History of 3D medical imaging 1969 – Hounsfield and Cormack develop the CT scanner 1970 – Greenleaf and colleagues report first biomedical 3D display; computer-generated oscilloscope images relating to pulmonary blood flow First commercial CT scanner introduced 1975 – Ledley and colleagues report first 3D rendering of anatomic structures from CT scans 1979 – Herman develops technique to render bone surface in CT data sets; collaborates with Hemmy to image spine disorders

Early History of 3D Medical Imaging 1980 – A CT scanner manufactured by General Electric features optional 3D imaging software 1980 – 1982 – Researchers begin investigating 3D imaging of craniofacial deformities 1983 – Commercial CT scanners begin featuring built-in imaging software packages 1986 – Simulation software developed for craniofacial surgery 1987 – First international conference on 3D imaging in medicine organized in Philadelphia 1990 – 1991 – First textbooks on 3D imaging in medicine published; atlas of craniofacial deformities illustrated by 3D CT images published –Taken from Seeram E; Computed Tomography Physical Principles, Clinical Applications, and Quality Control, 2001

Fundamental 3D Concepts The following rules should be followed when acquiring data sets –Field of view, matrix size, and centering must be the same for all images –Angulation/orientation must be the same for all slices –There should not be any duplicate images within the dataset –Thinner slices are typically better –Usually a “standard” algorithm is best for acquiring data sets – Edge algorithms are often too noisy

Fundamental 3D Concepts Resolution – 3D images and reconstructions appear best in the planes they were acquired 3D images in the acquisition plane have the same resolution as the original image set (256x256 or 512x512) 3D images in any plane other than the original data set, the resolution will depend on the inter-slice distance

Fundamental 3D Concepts When a voxel has the same dimensions in all planes, it is said to be isotropic –Isotropic voxels will allow the model to approximately the same resolution in all planes

Modeling The generation of a 3D object using computer software is called modeling Models can be rotated and viewed from many different angles Several modeling techniques exist –The most common is called extrusion –Extrusion uses computer software to transform a 2D profile into a 3D object –An example is a square being changed into a box

Modeling Several modeling techniques exist –The most common is called extrusion –Extrusion uses computer software to transform a 2D profile into a 3D object –An example is a square being changed into a box –Extrusion can also be used to create a wireframe model –Wireframes were more common in the early days of medical 3D, but are still commonly used in other applications

Wireframe Model of an Embryo

Modeling After the wireframe model is created, a surface is created by placing a layer of pixels and patterns on top of the wireframe The technologist can control various attributes such as color and texture

Shading and Lighting Shading and lighting help to add realism to the model Several different types of shading algorithms exist A few examples are: –Constant shading –Faceted shading –Gouraud shading –Phong shading Each technique has its own advantages and disadvantages

Constant Shading One shade or color is assigned to an entire object (

Faceted Shading Simple and quick but not very realistic (

Gouraud Shading Better than faceted, looks smoother (

Phong Shading Makes images appear smooth and shiny (

Rendering Final step in the process of generating a 3D object Rendering is a computer program that converts the anatomic data collected from the patient into the 3D image seen on the computer screen Rendering adds lighting, texture, and color to the final 3D image

Rendering Two types of rendering are used in radiology –Surface rendering: Uses only contour data from the data set. Creates an external surface that is hollow. Less memory intensive than volume rendering –Volume rendering: Uses the entire data set to create a 3D image. Produces a better image than surface rendering, but uses more computing power

Classification of 3D Imaging Approaches The primary approaches to 3D imaging have been identified –Slice imaging –Projective imaging –Volume imaging

Slice Imaging Simplest method of 3D imaging Also known as multiplanar imaging (MPR) Slice imaging doesn’t produce a true 3D image but rather a 2D image displayed on a computer monitor MPR is available on all CT and MR scanners MPR produces coronal, sagittal, and oblique images

MPR Oblique sagittal reconstruction

Projective Imaging Most popular 3D imaging approach Still doesn’t offer a true 3D model –Some people classify projective imaging as 21/2 D or 2.5D Projective imaging uses the axial stack obtained from a CT exam to create projections of what various anatomical structures would look like from many different angles

Projective Imaging Axial MRI of the circle of willis has been subjected to a projective imaging technique.

Projective Imaging Central kangaroo is projected at several different angles into the 2D viewing space

Volume Imaging Volume imaging should not be confused with Volume rendering Volume rendering (often seen in MRI and CT) is a class of projective imaging Volume imaging produces a true 3D visualization mode

Volume Imaging Various methods of volume imaging include –Holography –Stereoscopic displays –Anaglyphic methods –Varifocal mirrors –Synthanalyzer –Rotating multidiode arrays

Picture of a Hologram

Generic 3D Imaging System Four major elements are noted for any 3D imaging system –Input –Workstation –Output –User

Input Devices that acquire the data –CT scanner, MR scanner The acquired data is sent to a workstation

Workstation The workstation is the heart of the 3D system The workstation is a powerful computed that handles the various 3D imaging operations –Preprocessing –Visualization –Manipulation –Analysis

Output Once processing is completed, the results are displayed for viewing and recording

User The user interacts with each of the three components to optimize use of the system

4 Steps to Create 3D Images 1. Data acquisition – slices, or sectional images, of the patient’s anatomy are produced. Methods of data acquisition in radiology include CT, MRI, ultrasound, PET, SPECT, and digital radiography 2. Creation of 3D space or scene space. The voxel information from the sectional images is stored in the computer –Scene is defined as a multidimensional image; rectangular array of voxels with assigned values

4 Steps to Create 3D Images 3. Processing for 3D image display. This is a function of the workstation and includes the four operation listed above 4. 3D image display. The simulation 3D image is displayed on the 2D computer screen

Maximum Intensity Projection Maximum Intensity Projection (MIP) is a volume rendering technique that originated in magnetic resonance angiography and is now used frequently in computed tomography angiography. MIP does not require sophisticated computer hardware because, like surface rendering, it makes use of less than 10% of the data in 3D space

Steps Involved in MIP A mathematical ray is projected from the viewer’s eye through the 3D space This ray passes through a set of voxels in its path The MIP program allows only the voxel with the maximum intensity to be selected

Stand Alone Workstations Picker, Siemens, General Electric, and several other manufacturers provide 3D packages. Most workstations offer a variety of 3D processing features

3D Processing Features Multiplanar Reconstruction (MPR) –Can demonstrate the entirety of a curved anatomical structured in one image. This feature could be useful in demonstrating the entire length of the descending aorta in one view Surface Rendering Slice Plane Mapping –Allows two tissue types to be viewed at the same time

3D Processing Features Slice Cube Cuts –This is a processing technique that allows the operator to slice through any plane to demonstrate internal anatomy Transparency Visualization –This technique allows the operator to view both surface and internal structures at the same time

3D Processing Features Maximum Intensity Projection 4D Angiography –This shows bone, soft tissue, and blood vessels at the same time to allows the viewer to see tortuous vessels with respect to bone Disarticulation –This shaded surface display technique allows the viewer to enhance the visualization of certain structures by removing others

3D Processing Features Virtual Reality Imaging –Some workstations are capable of virtual endoscopy. This allows the viewer to “fly through” various anatomical structures including the colon and bronchus