Theory - Applied Measurement Techniques (Word)

ECG

 CT gantry – internal structure (3/4 generation)

a

tube is connected to the high voltage generator and heat exchanger (cooling system) through large

X-ray

cable machine

Heavy rotate around

Sensors system includes the detector, the cooling of detectors and data acquisition system

Acquisition

 CT MAIN SYSTEMS

 CT - Main systems are: imaging system;

computer system; display, recording, storage

system; data acquisition system

the acquisition, there is the

After

computational/ reconstruction system starting

from the raw data (coming from the sensors) to

convert the values of intensity into digital

values, then to filter the noises, finally the

matrix of data is used for the back reflection

algorithm

 Scanner: gantry and patient couch

-GANTRY HOUSES (hardware):

• X-RAY TUBE

• GENERATOR (LOW VOLTAGE DESIGN)

• COLLIMATORS/FILTERS

• DETECTORS

the gantry there are a large number of detectors that need to be able to produce analog signals, then

Within

are stored in a memory (S/H: sample and hold circuit -> interface between the analog signal and ADC) to be

digitalized through ADC (analog to digital converter), obtaining a set of bits for all the sensors and one

projection, bits giving to an array processor

of scanning controller: to control along the z-axis the gantry and patient couch both in case of single

Room

and multi-slices; at the same time it controls the high-voltage generator to apply to the X-rays tube

-GANTRY CHARACTERISTICS:

• APERTURE: 70 cm of diameter

• TILTING RANGE (of the table): +7-30°

• SCANNABLE RANGE: COVERAGE HEAD TO THIGH (162 cm) -> higher than 162cm means that there are

needed more than one scan

 IMAGING SYSTEM

1) Production of X-rays

2) Shaping of X-ray beam energy

3) Filtering X-ray beam - X-RAY TUBE

• X-ray tube for CT is similar in design to the

conventional radiography tube, but is specially

designed to handle and dissipate excessive heat units

(measurement unit: capacity of the system to remove

heat) – much higher heat loading

a device with a rotating anode and high heat

It’s

dissipating capacity

main issue is to regulate the enormous

The

production of heat

• Typical working conditions: 120kV, 200-500mA

• Ceramic target backing -> Decreases tube weight

time acquisition -> high exposition to radiation

Long

- GENERATOR –HIGH VOLTAGE

- COLLIMATORS

xy-plane: the post-patient collimation is needed in order to have radiation only addressed on the sensor

on

(limiting the area exposed to the sensor)

from analog to digital data -> stored in the PC

ADC: projecting from X-rays tube to patient there is the penumbra regions, that’s the reason why there

Whenever

are needed two collimators, used to reduce/eliminate the shielding of gray values because the X-ray sources

are not point light source, making them perfectly aligned 24/11

two collimators need to be mobile to regulate how large is the aperture angle and the angle of acquisition

The

of the sensors

the detector (hundreds of them) is able to detect a signal, then the projection signal is available at the

Once

analog level (continuous signal), then it is converted into digital value. The velocity by which download the

digital projection in the memory of processor determines the velocity by which I move the source around

patient

(pre-patient) collimator defines slice thickness while the second (post-patient) reduces scatter reaching

first

the detectors

- FILTER used to have an uniform beam projected to the patient

Filter

• Filtration material: aluminum of 1.5mm (special filter in CT placed on the X-ray tube) remove soft

to

radiation (low energy -> no enough to reach patient) vs hard (high energy)

radiations are those able to travel through the patient and reach the sensor -> to

create image

• filtration 

↑ ↓intensity ↑energy

• BOWTIE filter: to make the beam harder and more monoenergetic (uniform)

the thickness is not constant because at the center the energy of the beam is



typically lower than the side

xy there is a fan kind of emission (some angle to illuminate the patient), while along zy has to be as thin as

on

possible (to illuminate just the thickness of the sensor, no more because the radiation coming out the sensor is

travelling through the patient without creating any signal). The only case in which the section illuminating the

sensor can be enlarged is the multislice (more than one sensor along z-axis need to be illuminate)

- DETECTORS

• Function as image receptors for remnant radiation, then converts the measurement into an electrical signal

proportional to the radiation intensity.

• Two basic detector types are used: 1) Scintillation (solid state) detectors and photodiodes -> more

common

->(a) A solid-state detector consists of a scintillating crystal and

photodiode combination. (b) Many such detectors are placed

side by side to form a detector array that may contain up to 4800

detectors.

material/crystal is able to convert X-rays photons into an emission of optical photons (light) , so

Scintillation

they don’t need to be visible, but they are able to produce quantity of light that only photodetectors are able to

detect by producing a voltage signal

2) Ionization (xenon gas) detectors.

->Gas ionization detector arrays consist of high-pressure gas in multiple

chambers separated by thin septa. A voltage is applied between

alternating septa. The septa also act as electrodes and collect the ions

created by the radiation, converting them into an electrical signal.

(high absorbing metallic plate) separates each of this kind of

septa

detector one to the other, because if there are scattered photons in a

wrong direction, they more probably touch this metallic septa and do

not touch the scintillation crystal (-> stop noisy X-rays from supposed good one coming from the focus of the X-

rays through the patient directly on the sensor). Each of this detector has to be connected to an amplifier (so

either you consider hundreds of sensor to connect to single array of detector or if you know how many sensors

to illuminate (not all) then you have to multiplexed/connected time to time to the set of sensor), the only thing is

that you need to disconnect and reconnect to a new crystal when you move the source.

layout of classical radiology

Same

higher the number of sensors, the more detailed the projection signal, then the better the reconstruction

The optical photons are converted either by a photomultiplier PM tube (single device for converting light in V)

The

or photodiode (semiconductor component very well suitable to convert light into V -> more common

nowadays)

• Scintillation crystals used with PM tubes: sodium iodide – afterglow + low dynamic range (used in the past);

calcium fluoride;

bismuth germanate;

cadmium tungstate.

primary factor is the e iciency of conversion (how many optical photons we are able to produce from a

The

single X-ray photon, the larger is the number the higher the e iciency because of higher the signal from PM)

• Scintillating crystals used with photodiode: calcium tungstate

rare earth oxides – ceramic

• Gas ionization is an old fashion way to convert signal (X-rays directly into voltage)

are very large detectors (ionization chamber with Xenon gas at 30atm)

there

• e iciency of detectors – QDE: scintillation – 95/100%- commonly used in iii & iv generation scanners

gas – 50/60%

terms of patient health: launching 10 x-rays photon, they all travel the body but just 5/6 come out (become

In

signal) then the 4/5 cause damage

 DATA ACQUISITION SYSTEM (DAS) ->The data-acquisition system converts the electrical signal

produced by each detector to a digital value for the computer

is needed because of hundreds of sensors

’multiplexer’

interrogated, but not necessary all together

processor’ is motherboard that already starts to process

’array

the projection, in this way the host computer takes as input the

pre-processed data (no raw data) for reducing the computation

load

-DAS is a set of electronics between detectors and host computer. It contains:

1) AMPLIFIER: For signal magnification. ->increased SNR

signal detected by the detector is going to be the lowest as possible for reducing the patient damage

the

due to ionizing (-> so less part radiation possible is sent to the patient), having the lowest but enough

quality of signal

2) ADC: Converts analog signal output from the scanning equipment to a digital signal (operating with the

lowest quantization noise).

3) DAC: : Converts digital signal output from the scanning equipment to a analog signal.

4) GENERATOR: Power supply for the X-ray tube.

high-voltage generator produces the potential di erence between anode and cathode; it requires

the

brush contacts to allow at the same time the rotation and guaranteeing the supply

5) S/H: Located between amplifier and ADC performs sampling and assigns shades of gray to the pixels in the

digital matrix corresponding to the structures.

layout is required for each sensors that we’re illuminating description conclusion

This hardware

 HOST COMPUTER SYSTEM

-RECONSTRUCTION AND POSTPROCESSING the bac projection algorithm on projection acquired

applying

-CONTROL OF ALL SCANNER COMPONENTS of the scanning process (velocity, how many sensors

control

illuminating…), that control in turns the power and angle of X-rays tube to limit the exposition of the patient

-CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY

-DATA FLOW DIRECTION the information are putted/stored

where

-Typical operating system: UNIX (pay) or WINDOWS (free) -> able to perform parallel tasks

-Computer processing in CT (requirements): SEQUENTIAL PROCESSING, MULTITASKING, MULTIPROCESSING

-ARRAY PROCESSOR: takes detector measurements form hundreds/thousands of projections. It’s responsible

for retrospective reconstruction and postprocessing of data

->the more processors in the computer, the shorter the reconstruction time

there are needed semi-real time or real time reconstruction, then the tasks have to be run very fast

If

 DISPLAY, RECORDING, STORAGE SYSTEM

-Display Field-of-View (DFOV)

• Scan Field of View (SFOV), how much anatomy is scanned. ( always lower than the gantry diameter, 70cm)

defined by the angle of aperture, so it’s the area illuminated by X-rays tube and filling a part into the gantry

it’s

• It is always larger then anatomy.

• Displayed Field of View (DFOV), how much of the SFOV is reconstructed.