Computed Tomography RAD309

CT Components

Three Major Systems

• Imaging System• Computer System• Image Display/Recording/Storage System

• Each system in a separate room

Major Components

• Scanner room– Imaging system

1. gantry assembly2. patient table

• Computer/Electronics Room– Power– Computer– Generator

• Operator’s Area– Display / recording / storage– Computer

Imaging System

Propose:1. To Produce x-ray2. Shapes & filters (hardens) beam3. Detect (measure) radiation passing

through patient4. Convert transmitted photons

– Provides digital intensity data to computer

Imaging System

Components:• X-ray tube X-ray Beam• Generator• Collimator slice thickness, restricts beam

• Filter Protect Patient, Uniform beam-Detector

• Detector capture photon, convert to elect.signal

• Detector electronics (DAS) convert to digital

Gantry Assembly

• All previous components in Gantry Assembly

• Moving Frame, Tilt, Gantry aperture

• Rotational components including slip ring , positioning laser lights,

Gantry: Components

• X-ray tube– high heat capacity– modern tubes very large & heavy– external cooling (oil heat exchanger)– high speed rotation of the anode– Heat units: heat capacity of 3.5 to 5 million

heat units (MHU)– focal spot size of an x-ray tube is determined

by the size of the filament and cathode which is determined by the manufacturer

Gantry: Components

• Generator, inside gantry– supplies power to x-ray tube

• filament• high voltage

– Different power 30-60Kilowatts– Allowing range of exposure techniques:

80,100,120,130,and 140kV and 30,50,65,100,125,150,175, and 200,mA

Gantry: Components

• Collimators– Tube/Source collimators: restricts beam to

slice of interest– Pre-patient collimators: maintain the width of

the beam as it travels toward the patient – Post-patient/Pre-detector collimators: below

patient and above detector proper beam width at detector and reduce no. of scattered photons at detector

Gantry: Components• Filter

– protects patient from low-energy photons– provides a beam closer to mono-energetic– provides a more uniform beam (hardening)

• Mathematical filtration: bone/tissue algorithms (recons. Process)

• Inherent tube filtration • Filters made of aluminum or Teflon; to shape

beam-low energy photons and scatter

Gantry Components

• Detector = image receptor• beam travels through patient and

attenuates by the anatomical structures it passes through

• Job: collect attenuated photon energy and convert it to electrical signal, which will then be converted to a digital signal for computer reconstruction

Detectors• Crystal or ionizing gas • When struck by x-ray photon , produces light or electrical

energy• Two types : scintillation or solid state and xenon gas

detectors

• Scintillation detectors- uses a crystal that fluoresces when struck by x-ray photon which produces light energy– A photodiode, attached to scintillation portion, transforms light

energy into electrical or analog energy– Strength of the detector signal is proportional to number of

attenuated photons that are successfully converted to light energy and then to an electrical signal

A Note on Detectors• Detector: single element of detector array,

produces electrical/light signal as a response to x-rays

• Detector array: total number of detectors that a CT system utilizes for collecting attenuated information

• A Ray: path that an x-ray beam travels from tube to a single detector

• Detector reads each ray and measures resultant beam attenuation

Cont.

• Ray Sum: attenuation measurement of each ray

• Projection (view): a complete set of ray sums – It takes many projections to create a

computed tomography image

Gantry Components• Detector electronics, data

acquisition system (DAS)– Receives weak signal form detector– includes A-D converter– Amplification

• provides digital information to reconstruction computer

Gantry Requirements

• Patient size• size of room• Patient aperture

from 50-85cm• Tilt range +/-30°

Patient Couch

Strong, safe, and hygienic, made of radiolucent material

1. Couch top: carbon fiber composite, low absorption

2. Pedestal: mechanical and electrical components

Patient Couch• Vertical travel

– minimum height for stretcher transfer• Horizontal travel

– reduces need for re-positioning patient– accuracy

Patient Couch Requirements

• Radiolucent• Easy to clean• Weight limits: 450lb• Total travel: 180cm• Accuracy: +/- 0.25mm• Speed: 25mm/sec

Computer System

• Receives digital data from DAS• Reconstructs cross-sectional image• Image display, manipulation, & processing

– windowing– enhancement– zoom– quantitative measurements– multi-planar & 3D reconstruction

Computer System Components• Input / output

devices– Detectors– Printer

• Central processing units (CPU’s)

• Array Processor– Reconstruction

Hardware

Interfaces to other computers• Remote service• RIS• Archive• Web browser

Communications hardwareSoftware• enables above hardware

operation• image manipulation• scanning control

Computer System

• Processing architecture• Hardware • Software

Processing Architecture

Capable of Multi-processing – Scanning, reconstructing, archiving

Three Types: 1. Pipeline processing architecture

– various reconstruction steps may be performed by separate processors

– Very fast image reconstruction, after scanning image can be viewed

– Overlapping moving data into pipe

Processing Architecture (cont.)

2. Parallel processing architecture– 3 multi-task computers coupled– More than one CPU to execute program– Coordination, dividing program instructions– separate scan, display, & file processors

3. Distributed processing architecture– many independent processors– each controls specific function– each runs a different activity

CT Software Functions

• Operator console– Allows operator to

communicate with computer & scanner

– keyboard– touch screen– mouse / trackball

• Control of scanner– Gantry tilt– scout / axial– field of view– x-ray technique– slices

• how many• thickness• overlap

CT Software Functions

• Control of imaging– selection of pre-stored image protocols– Selection of reconstruction algorithm

• Operating System– Control hardware of computer– often UNIX

• multiuser• multitasking

Image Display, Record, Storage

Summary: • Display output image• Provide permanent hard-copy for

– radiologist interpretation– referring physician– archival

• Facilitate storage / retrieval of digital data

Image Display

• Display– CRT or flat panel

• images in gray shade• non-image data in color or B/W

– often hi-resolution• Can display 4-on-1 or zoomed image at full

resolution

• Display processor provides– display functions– image manipulation

Image Storage

• Storage:• 512x512x2 bytes (16bits) deep• Requires 0.5MB storage• 1 Examination= 50 images, 25MB• 50/day = 1.25GB

Image Size

• 512 X 512 image = 262144 pixels

• 262144 pixels X 2 bytes / pixel = 524288 bytes per image ~ 512 KB ~ 0.5 MB

High Capacity Image Storage

• Magnetic disk• Magnetic tape• Digital videotape• Optical disks• Optical Tape

• Capacity• Access Speed• Rewritable?• Size / space• $$

Types Considerations

CT Hard Copy on Laser Printer

• Requirements for hard copy– broad gray-scale contrast

resolution– high spatial resolution

• Two types :• Solid state and gas lasers

Laser Printing• Unexposed film will move to exposure

region, film exposed by scanning line by line with laser

• Producing a latent image

CT Console

• Keyboard– Alphanumeric,

special function keys, trackball, windowing controls

• Display– CRT– Flat panel– Touch panel

CT Console Allows Operator to Control

• Gantry tilt• scanning parameters

– Slice thickness– Spacing– Technique

• image manipulation– multiplanar reformatting– 3D– zoom / pan

Control Functions

• Archiving• Voice instruction to patient• Visual instructions to patient• Filming protocols

– Window / level (for contrast)– Format (12 on 1 for example)

Options and Accessories

Hardware:• Archival systems• Independent workstations

– Physicians– 3D

• Printing– Laser– Multi-format

CT Options

• Software:• Bone mineral analysis• Dynamic scan

– rapid sequential scans of single slice• Multiplanar reformatting• 3D image reconstruction• Regional cerebral blood flow analysis• CT Angiography• Dental CT (surgery planning, implants)• Networking

Accessories

• Support – immobilization• Pads, wedges, head rests, straps,

mattress, knee support pad

CT Room Layout

Scanning Room:• Gantry, Patient Couch• Room for

– Resuscitation team– Emergency equipment– Stretchers

• Medical Gasses• Shielding

CT Room Layout

Equipment room/Computer– Host computer and computing equipment

• Control room– Control console, film recording equipment

Purchasing Specifications

• List of Items– Major components– Optional equipment

• Major technical features– should be measurable

CT Specifications Who’s Involved?

• Radiologist• Technologist• Physicist• Biomedical Engineer• IT • Engineering Management (new)

Technical Specifications

• X-ray generator parameters– operational parameters

• maximum kVp• kW rating

• X-ray tube– heat capacity– cooling rates

• Detector efficiency

• Reconstruction time

CT Specifications• Gantry

– aperture size– tilt range– laser positioning aids– controls

• Couch– strength / weight limits– movement characteristics

• Speed• Limits

CT Specifications Operator’s Console

• Display, keyboard, mouse, touch panel characteristics

• Software design– Choice box of available

options vs. manual typing• Film recording &

archival considerations

CT Specifications• Physician’s console & other

workstations– functions available– speed– Simultaneity

• Computer– storage requirements– CPU– networking

CT Specifications• Interfaces• PACS hooks & DICOM compatibility

– Worklist management• Pulling patient demographics from RIS

– Image storage• Transfer of images to archive / PACS

Computer Software CT Specifications

• image reconstruction options– FOV– filters

• display options– annotation

• analysis options– regions of interest

CT Specifications• Hardcopy devices &

interface– laser printer

• Quality control equipment– phantoms– software

• Remote Diagnostics

CT Specifications• Acceptance Testing

– Who performs– What tests done

• Technology Changes– “Latest model available at

time of delivery”• Service

– Warranty– Binding service contract

prices

Room Requirements

• Scanner Room• Console Area• Computer Room• Design

CT Specifications Training

• For whom?– Technologists– How many?

• Where?– Vendor site– On-site– Local

• What’s included– Registration– Books– Travel expenses