CT Imaging Fundamentals

CT Basics

CT eliminates superimposed structures, differentiating small density differences in anatomy and abnormalities. Images are cross-sectional, like a loaf of bread, showing anatomy at a specific level. Thickness is the z-axis, width is x (right/left), and height is y (anterior/posterior).
Pixel = picture element, voxel = volume element (3D data).
Scout setup involves setting couch zero, start location, and zeroing the couch.
ODM (Object Data Module) setup involves Z-axis only, a maximum of one per group, indicated on kV/mA settings and reduce dose.
SFOV (Scan Field of View) is the imaged anatomy's dimensions, and should just exceed the anatomy being scanned (small, medium or large SFOV).
DFOV (Display Field of View) can be less than or equal to SFOV, but never more. Changing DFOV alters pixel values, and affects resolution. This assigns a pixel value in image reconstruction.
Noise index (NI) balances image quality and patient dose, NI=noise/mA and dose, and corresponds to the level of noise on the reconstructed image.
Simulated images show low noise, high dose and high noise, low dose.

Image Reconstruction

Fourier Transform: used to study waves; used in image analysis.
Filtered back projection: Method to compile attenuation profiles to create images.
Interpolation: estimation of a value based on surrounding values to fill in missing data in the reconstructed image.
Raw data: measurements from detectors, unprocessed scan data, not useful without reconstruction.
Image data: Processed raw data, where each pixel is assigned a Hounsfield unit value based on tissue density.

Lecture Two

CT uses multiple projections around the body to view different ways.
Cross-sectional images: each slice shows anatomy at a particular level.
Each slice shows a particular plane of the anatomy in the body.
Warm-up the tube and do testing (fast cals) to ensure detector functioning is optimal.
Digital modality assigned to correct the patient.
Based on protocol, for anatomy/ physiology.
Scout is taken. A 2D image resembling a plain radiograph (AP and LAT).

Lecture Three

X-rays first discovered in 1895.
J.H Radon developed the math equation for reconstructing cross-sectional images in 1971.
Godfrey Hounsfield developed the first CT scanner in 1973.
Kalender and Vock created spiral/helical CT in 1989.
First multi-slice scanners debuted in 1998.
Computed axial tomography (CAT), limited to axial cuts.

Generation of Scanners

First generation: Pencil beam, single or two detectors. Imaging head only. 5-minute imaging time.
Second generation: Fan-shaped x-ray beam. Multiple detectors. Head and body imaging. 3-minute 30-second imaging time.
Third generation: Fan beam, continuous rotation. Curvilinear detectors. 1 second imaging time.
Fourth generation: Fan beam, stationary detectors. Very costly.
Fifth generation: Electron beam CT. Very fast but limited spatial resolution.
Sixth generation: Dual Source CT (DSCT). Improved and maintained spatial and temporal resolution. Cardiac imaging.
Seventh generation: Flat-panel detectors. FPCT uses third gen, quick and great spatial resolution but lesser contrast resolution.

Lecture Four: Data Acquisition

Five main components of the gantry: tube housing, generator, filters, collimators, and detectors.
Pre-slip rings: early CT scanners, recoil system cables. Stop-and-shoot method, limited gantry rotation times.
Slip rings: current systems: electromechanical devices and electrical brushes. Allow for continuous electrical power supply.
X-ray source: high-performance x-ray tubes, stress on CT tubes with long exposure time, high-speed RPMs (6000-10000) and large diameter/thickness of anode discs.
Filtration: decreased radiation dose to patient.
Collimation: slice thickness size (0.5-20mm), influencing detector configuration, and SFOV.
Detector characteristics: good QDE, low afterglow, and high scatter suppression, high stability.
Optimal values allow for quick functioning and better image quality.

Lectures Five and Six: Image Reconstruction and Display

DFOV (Display Field of View): zoom/ target, determines how much raw data is used to reconstruct images.
DFOV cannot be larger than SFOV (Scan Field of View).
Small DFOV: small tissue area presented from the data
Large DFOV: all tissue data is presented in a condensed way.
Cartesian coordinates, or RAS coordinates (R/L, S/I, A/P) are used to specify locations in reconstructed images.
Image reconstructions involving scan data previously gathered.
CT scan image, reconstruction in different planes (axial, coronal, sagittal) and 3D.

Lectures Seven and Eight: The Use of Imaging in Radiation Therapy

CT sim: modified CT scanner for 3D images.
Conventional sim: mimics a treatment unit, captures 2D images (primarily bony anatomy).
Commissioning: electron density tables generated by physicists; used to correlate CT numbers to tissue densities.
QA tests (daily and monthly): ensure proper functioning of the machine and consistency
Tabletop: flat with low density (similar to air) in order to mimic the radiation treatment bed with ease and accuracy.
Helical mode: gantry tilt is never applied.
Extended field of view (EFOV): used to delineate body contour and accurately reconstruct the CT in cases where the body structure extends further than the detectors' range.
Oncology-specific protocols: breathing techniques.

Lectures Nine and Ten: Image Display and Methods of Data Acquisition

Spatial resolution: ability to distinguish fine details in images, high contrast resolution.
Contrast resolution: ability to differentiate between objects with similar densities
Temporal resolution: how rapidly data is acquired, quicker scans are better
Localizer scans: used to acquire scout images to guide other scans.
Step and shoot: used for scanning, generating images that are perpindicualr to the Z-axis (axial) but parallel to every other slice.
Multiple image display: several images displayed for comparison and analysis.
Histogram: graphical representation of CT values within ROI (Region of Interest).
Graphic display: graphical representation showing frequency of CT values within ROI.
MPRs/3D Reformatting: used to create coronal, sagittal, curved or oblique images from axial images, and used in analyzing complex anatomy.

Lectures Eleven to Fourteen: Contrast Imaging

Contrast Medium: Used to enhance contrast between tissues.
Contrast media types: iodinated (water-soluble), barium sulphate (non-water soluble), air, or water.
Gastrointestinal: oral, rectal contrast medium, intravenous, intrathecal and intra-articular.
Advantages of performing contrast mediums in vascular scans.
Important factors to consider when evaluating contrast studies or scans.
Techniques to prevent extravasation (e.g., large gauge IV, appropriate flow rates, and monitoring pressure with a power injector).
Normal and unfavorable reactions are considered when performing contrast studies as well.

Lectures Fifteen to Seventeen: Venipuncture Injection Techniques and Artifacts in Image Formation

Peripheral IVs (e.g., 18-22G): various gauges of catheters with consideration given to the size of vein in patient, use of metal needles and plastic catheters. Power injectors are used with these techniques, that facilitate quick administration for contrast material application into patient in an appropriate manner.
Factors to avoid: scarred areas, edema, infection and cellulitis at the ROI, the same side/site of mastectomy or radiation, and the site patient preference.
Appropriate steps for accessing suitable vein locations.
Preparing site for insertion (e.g., cleaning and prepping skin, appropriate angles for needle delivery and tissue puncture and proper placement of tourniquet).
Complications of IV administration: infection, extravasation (contrast leakage into surrounding tissues), superficial thrombophlebitis (blood clot in the vein), nerve damage.
Contrast mediums and effects if not monitored appropriately.

Lectures Eighteen to Twenty: Radiation Dose, CT Phantoms, and Dose Measurement

CT dose: Major concern, choosing wisely. Conventional Radiography dose approximations.
How does dose distribution differ from general radiographs, and how is the absorbed dose in the CT scan measured?
CTDI (computed tomography dose index), (CTDI)W
MSAD (multiple scan average dose), DLP (dose length product).
Phantom testing techniques are used to determine if the scanner is operating according to specifications and expected standards.

Other Lectures: Additional Information

Image manipulation: Reconstruction (manipulating original data to create images) vs. reformatting (rearranging image data to create images in new planes or 3-D).
Visualization tools, Multiple imaging windows (WW/WL), Distance and angles, Highlighting, Image magnification, Coordinates for biopsies, Image histogram, and ROI measure HU.
Advanced tools: require powerful computers inc. MPR, 3D surface rendering, VR volume rendering, Max intensity projection, Min IP, etc.
Choosing appropriate technique for various anatomical and clinical situations.