CT scan

CT scanIntroduction

Tomos is the Greek word for cut or section, and tomography is a technique for digitally cutting a specimen open using X-rays to reveal its interior details. A CT image is typically called aslice, as it corresponds to a slice from a loaf of bread. This analogy is apt, because just as a slice of bread has a thickness, a CT slice corresponds to a certain thickness of the object being scanned. Therefore, whereas a typical digital image is composed ofpixels(picture elements), a CT slice image is composed ofvoxels(volume elements).

First developed for widespread use in medicine for the imaging of soft tissue and bone, X-ray CT was subsequently extended and adapted to a wide variety of industrial tasks.Introduction

They employ the following optimizations:Use of higher-energy X-rays, which are more effective at penetrating dense materialsUse of smaller X-ray focal spots, providing increased resolution at a cost in X-ray outputUse of finer, more densely packed X-ray detectors, which also increases resolution at a cost in detection efficiencyUse of longer exposure times, increasing the signal-to-noise ratio to compensate for the loss in signal from the diminished output and efficiency of the source and detectors.Introduction

Figure 1

Figure 1: Schematic illustrations of different generations of X-ray CT scan geometries. Solid arrows indicate movements during data collection, dashed arrows indicate movement between sequences of data collection. The solid lines passing from the sources to the detectors are ray paths, and each set of solid lines from a single angular orientation constitutes a view. These illustrations show the source and detectors moving around a stationary object, as is the case with medical scanners. The motion is relative, however, and in many industrial scanners the object moves while the source and detectors are stationary. In all cases, the axis of rotation is the center of the circle. A. First-generation, translate-rotate pencil beam geometry. B. Second-generation, translate-rotate fan beam geometry. C. Third-generation, rotate-only geometry. D. Third-generation offset-mode geometry.

Basic CT scanner components Gantry X-Ray Tube Detector Control Console

Basic CT scanner components

Gantry CT X-ray tube High voltage generator Detector array Data acquistion system Slip ring

Basic CT scanner components

The CT X-ray Tube Anode heat capacity 3.5 MHU up to 6.5 MHU Determines maximum mAs Determines volume length Dictates generator size

Basic CT scanner components

Detector Elements Capture energy that has not beenattenuated by the patient

Basic CT scanner components

DefinitionsA computed tomography (CT) scan is an imaging method that uses x-rays to createpictures of cross-sectionsof the body. CT scans are also sometimes known as CAT scans, or computed tomography scans.

Uses of CT scanCT scans can be used to diagnose and monitor a variety of different health conditions:brain tumourscertain bone conditionsinjuries to internal organs such as the kidneys, liver or spleenused to look at the heart

Your doctor may recommend a CT scan to help:Diagnose muscle and bone disorders, such as bone tumors and fracturesPinpoint the location of a tumor, infection or blood clotGuide procedures such as surgery, biopsy and radiation therapyDetect and monitor diseases and conditions such as cancer, heart disease, lung nodules and liver massesMonitor the effectiveness of certain treatments, such as cancer treatmentDetect internal injuries and internal bleeding

Uses of CT scanRisks

Radiation exposureDuring a CT scan, you're briefly exposed to ionizing radiation. Harm to unborn babiesReactions to contract materialIn certain cases, your doctor may recommend you receive a special dye called a contrast material through a vein in your arm before your CT scan. Although rare, the contrast material can cause medical problems or allergic reactions.

Scanning Methods

Digital projection AP, PA, Lat or Oblique projection Surview, Scanogram Conventional CT Axial Start/stop Volumetric CT Helical or spiral CT Continuous acquisition

Scanning Methods

Digital Projection

X-ray tube and detector remain stationary Patient table moves continuously With X-rays on Produces an image covering a range ofanatomy Similar to a conventional X-ray image, e.g. flat plate of the abdomen Image used to determine scan location

Scanning Methods

Axial CT X-ray tube and detector rotate360 Patient table is stationary With X-rays on Produces one cross-sectionalimage Once this is complete patient ismoved to next position Process starts again at thebeginning

Scanning Methods

Volume CT X-ray tube and detector rotate 360 Patient table moves continuously With X-rays on Produces a helix of image information This is reconstructed into 30 to 1000 images

Scanning Methods

Advantages of Volume CT More coverage in a breath-hold Chest, Vascular studies, trauma Reduced misregistration of slices Improved MPR, 3D and MIP images Potentially less IV contrast required Gapless coverage Arbitrary slice positioning

Scanning Methods

Fundamentals of Multislice CT

Scanning Methods

Multislice Fundamentals Everything is better (R)esolution Z-axis, spatial, low contrast (S)peed Temporal bolus capture, stopped motion (V)olume Thin slice - organ-specific coverage (P)ower Enough photons uncompromising image quality

Scanning Methods

Scanning Methods

Calibration

Calibrations are necessary to establish the characteristics of the X-ray signal as read by the detectors under scanning conditions, and to reduce geometrical uncertainties. The latter calibrations vary widely among scanners; as a rule flexible-geometry scanners such as the ACTIS scanner at UTCT require them, whereas fixed-geometry scanners geared towards scanning a single object type may not.

NSI Scanner

The NSI scanner comprises two X-ray sources and within a single radiation-safety enclosure. Of the two X-ray sources, one yields ultra-high-resolution data on small objects that can be penetrated by relatively low-energy X-rays; the other yields high-resolution data on larger or denser objects that can be penetrated only by higher-energy X-rays.

NSI Scanner