Lic Imagenologia

CHAPTER ONE

CT Physics and Instrumentation – Mechanical Design
Jimmy Saunders and Stefanie Ohlerth

Basic CT Unit Anatomy
A computed tomography (CT) unit consists of a gantry, a patient table, hardware equipment, an operator console and optionally additional workstations. The gantry is a doughnut-shaped ring containing the X-ray tube, the detector array and associated equipment. Thecentral hole in the gantry accommodates the patient on a sliding table. The X-ray tube rotates around a slice of patient anatomy. This slice represents the X-Y plane, with the X-axis being horizontal and the Y-axis vertical. The isocenter of the gantry is the central point of this plane. The third dimension is represented by the Z-axis, which is along the orientation of the patient table. The patientbed is a sliding tray on a fixed table with an adjustable height and a defined capacity of forward motion. The operator console is located in another room or behind radioprotective screening, and allows operation of the CT units. Additional workstations can be used to review processed image data, but usually not raw data processing.

CO

electrons when heated. The anode consists of a disk oftungsten or a tungsten alloy with an annular target, called the focal track, close to the edge. The anode disk is supported on a long stem that is supported by ball bearings within the tube. The anode can be rotated by electromagnetic induction from a series of stator windings outside the evacuated tube. The X-ray tube is enclosed in a housing unit filled with insulating oil. This oil provides electricshielding from the tube voltage, X-ray protection and transmits heat generated in the housing unit to the unit’s surface. The exterior of the housing unit is cooled with a fan, and insulating oil is cooled by passing it through a heat exchanger. Low-power applications use stationary anode tubes, while for most mid-range and high-performance applications there is a need to utilize rotating anodetubes.

PY R

IG

HT
1

X-Ray Tube
Basic anatomy of the X-ray tube
An X-ray tube is a vacuum tube that produces X-rays. It is composed of a cathode (filament) and an anode (target). The cathode cup is negatively charged and incorporates a wound tungsten filament that emits

Veterinary Computed Tomography, First Edition. Edited by Tobias Schwarz, Jimmy Saunders. © 2011 John Wiley & SonsLtd. Published 2011 by John Wiley & Sons Ltd.

ED

MA

Basic physiology of the X-ray tube
A current of a few amperes (4–8 A) heats the tungsten filament that releases electrons (thermoionic emission) in the vacuum. A high-voltage power source (‘tube voltage’) ranging from 30 to 150 kilovolts (kV) is connected across cathode and anode to accelerate the electrons producing an electron flow (‘tubecurrent’). These electrons collide with the anode material and about 1% of their kinetic energy is converted into X-rays, usually perpendicular to the path of the electron beam. The remainder of energy is converted into heat, causing the

TE

RI

AL

2

V E T E R I N A RY C O M P U T E D T O M O G R A P H Y

X-ray tube to warm up during operation. The temperature of the focal trackcan increase quickly to 1000– 1500°C. Heat diffuses by conduction throughout the anode body and by thermal radiation (infrared radiation) to the tube housing (80%). Heat is removed from the tube housing by convection to the surrounding atmosphere. Many X-ray systems, including CT, have built-in safety features that will not allow the equipment to be operated in ‘overheated’ conditions. Thetemperature cannot be measured directly in the focal track. It has to be evaluated based on indirect values that characterize the ability of the anode to store the heat generated during the X-ray emission, such as the anode heat capacity, the anode dissipation/cooling rate or the tube dissipation.

Specificities for CT
Since the invention of CT, its demands regarding the X-ray source never ceased to...