医学成像技术-lecture2x-ray parti generationdose

ReviewMedicalImaging

enables…Medical

Imaging

changes

thehealthcare

comple

y…History

and

trend

of

MIT.Principle

of

medicalimaging.Five

modalities,superiority

&weakness210/29/2021310/29/2021ContentsX-ray

generation

equipment(Tube)X-ray

generation

physics

(Energy

spectrum…)X-ray

tube

focal

spotand

rated

parameterX-ray

radiation

doseReference:1.

<<医学成像的基本原理>>P22-P43410/29/2021Energy

and

RadiationBasicconcepts:Energy

is

an

objective

existence

in

the

universe,

it

can

changefrom

one

form

to

another

form,

but

can

not

be

eliminated

orcreated

inexplicably,

which

is

the

law

of

energy

conservation.SI

unit

of

energy

is

(J),

but

(eV)

is

commonly

used

in

micro.1eV

1.6021019

JRadiation

is

the

transfer

of

energy

in

space.Non-ionizing

radiation

(low

energy): heat

effectIonizing

radiation

(high

energy):

atom->

positive

ion

&

anionX-ray(X射线、X光）510/29/2021Basic

concepts:X-ray:

A

kind

of

EMwaveSoft

X-ray:

8nm(150eV)

~

0.08nm(15keV)Hard

X-ray:

0.08(15keV)nm

~

0.008nm(150keV)610/29/2021PartIX-ray

generation

equipmentX-raytube710/29/2021X-raytubeFour

necessary

elements:Electron

sourceHigh-field

electric

fieldProperVacuum

environmentsX

rays

are

produced

as

the

electrons

interact

with

the

.A

heated

filament

(灯丝)

releases

electrons

that

are

acceleratedacross

a

high

voltage

onto

a .

The

stream

of

acceleratedelectrons

is

referred

to

as

the

tube

current

(管电流).A

vacuum(真空)is

maintained

inside

the

glass

envelope

of

the

x-raytube

to

prevent

the

electrons

from

interacting

with

gas

molecules,

andprotect

the

filament.>99%---heat810/29/2021CathodeTungsten

filament,

melt

point

3370oC.Current

flow

J

(A/cm2)(灯丝发射电子的电流密度)J

aT

2eW

/

kTT-

Absolute

temperature

(unit:

K/Kelvin)W-

Required

energy

to

remove

one

electron,

4.5

eV

fortungstenk

–

Boltzmann

Constant,

1.38e-23

J/Ka

–

Constant,

120A/cm2K2Filament

current

Temperature3.

Filament

sh &

diameter

(linear&spiral)靶材料Tube

current

X-ray

intensity4.The

filament

is

mounted

within

a

negativelychargedfocusing

cup

（凹面聚焦阴极体）.钼Molybdenum

focusing

cup

function:focusing

&

prevent

secondary

electrons910/29/2021Anode1. Why

usetungsten

ormolybdenum?High

melt

point(靶上轰击点温度2600~2700oC)High

atomic

number

Z,

Mo-42;W-74;X-ray

intensity

is

determined

by atomic

numberK

–

Constant,

1.1e-9~1.4e-9I

–

TubecurrentV–

TubevoltageStable

&

rotate

2800~8500

r/minFocalspot

size(散热-图像分辨率)and

filament

shOther

heat

transfer

enhancement

techniquesNew

Rhenium(铼）&tungsten

alloyCarbon

&tungstenHollow with

oilcoolingx-rayI

KIZV

21010/29/2021Envelopeand

housingCutaway

of

a

rotating-anode

x-ray

tube

positioned

in

its

housingThe

tubeismounted

inside

ametal

housing

thatisgrounded

electrically.Shockproof

cables

that

deliverhigh

voltage

tothe

x-ray

tubeenter

the

housing

throughinsulated

openings.A

bellows

inthehousing

permitsheate l

to

expandwhen

the

tube

isused.Oil

surrounds

the

x-raytube

to

(a)

insulatethehousing

from

the

highvoltage

applied

to

thetubeand

(b)

absorb

heatradiated

from

the

anode.A

lead

sheath

insidethemetal

housing

attenuatesradiation

emerging

fromthe

x-ray

tube

inundesireddirections.Placing

thebeamcollimators

as

close

aspossible

to

the

x-ray.1110/29/2021Two

accessories:

collimator

and

filterX-raytubeCollimatorFilter

&

gridFilm,

IP

or

photodiode滤线栅的栅比1210/29/20215.5

A4.5

A3.5

A2.5

A管电流、管电压和灯丝电流间的关系The

filament

current(灯丝电流)is

the

flow

of

electrons

through

the

filament

toraise

its

temperature

and

release

electrons

（A）.The

flow

of

released

electrons

from

the

filament

to

the

anode

across

the

x-raytube

referredto

as

the

tubecurrent(管电流),varies

froma

fewtoseveralhundred

milliamperes

(几毫安~百毫安).I

increases

with

V,

but

keeps

constantwhile

V

reaching

a

saturated

value.Temperature

or

Filament

emissionlimited

温度或灯丝发射限制I

does

not

increase

with

i

while

V

isvery

low.Space-charge

limited

空间放电限制Saturated

V

increases

with

i.1310/29/2021Electricalcharacteristicsof

X-raytubeElectricalcharacteristicsof

X-raytubemA

·s

(tube

current

×

exposure

time)

is

used

to

calculate

the

totalnumber

of

the

electrons

interacted

with

the

.1A

=

1C/s

(1库仑/秒)，

1

Electron

charge

=

1.6e-19C试计算1mA

·s对应多少电子作用于阳极靶？X-ray

photons

∝

mA

·sEg.

A

X-ray

tube,

tube

current

is

200mA,

exposure

time

is

0.1s,

then

thetotal

electrons

interacted

with

the

is:Electrons

=

200mA

×0.1s

×6.25

×10e15electrons/mA

·s

=

1.25

×10e171410/29/2021High

Voltage

GeneratorThe

intensity

and

energy

distribution

of

x

rays

emerging

from

an

x-ray

tube

are

influenced

by

V.(强度和能量分布)The

potential

difference

(voltage)

between

the

filament

and of

thetube.Step-up

transformer

(升压变压器)

，220V->tens

Kv

to

hundreds

of

KvHigh-current

low-voltage

to

high

voltage

and

low

currentAlternatingcurrent

(AC)Direct

current

(DC)X-raypreferredmost

efficientProblem

of

ACAt

high

tube

currents,

however,

the

heat

generated

in

the can

be

greatenough

to

release

electrons

from

theflow

across

the

x-ray

tube

when

thesurface.

In

this

case,

electronsis

negative

and

the

filament

ispositive.

This

reverse

flow

of

electrons

canDESTROY

thex-ray

tube.1510/29/2021Voltage

waveforms

–

rectificationDiode(二极管)takesthe

critical

roleduring

therectification.Modern

solid-state

voltage-switching

devices(固态变压器)are

capable

of

producing“highfrequency”waveforms(高频波)yielding

thousands

of

x-ray

pulses

per

second.Half-wave

rectification1610/29/2021Full-wave

rectification1&3

phase

rectification1

pulse

per

cycle2

pulse

per

cycleCurrent

bridgeNearly

constant 50

Hz

1710/29/2021Part

ⅡX-ray

generation

physicsEnergyspectrumof

X-rayBremsstrahlung

radiation轫致辐射（刹车辐射）Characteristic

radiation标识辐射1810/29/2021Radiation

I

-BremsstrahlungV1>

or

<V2?The

velocityofinciden ectron

decreases

at

the

electric

field

of

atom

nucleus,and

the

energy

is

released

by

the

type

of

EM

wave(photon,

X-ray).Bremsstrahlung

轫致辐射/刹车辐射ectronV1Inciden入射电子Photon,

X-rayectronV2Emergen出射电子轫致辐射：泛指高能带电粒子与靶原子或原子核发生碰撞时突然损失能量发生的能量辐射；轫致辐射的强度与靶核电荷的平方成正比，与带电粒子质量的平方成反比；大原子序数的靶材料有助于形成强的轫致辐射；重粒子产生的轫致辐射远小于高能电子产生的轫致辐射。1910/29/2021Bremsstrahlung

energyspectrumkVp

–

peak

kilovlotageAngstroms–10-10

m,

埃;minmaxmaxE

h

hcLimit

wavelength(最短波长，对应最高能量光子)E

e

Vhcmin

eVExample:

50

kv,

limit

wavelength&frequency?Unit？Continuous

X-raySpectrum. Three

reasons:

The

velocity

change

of

electron

iscontinuous;

Different

depth;

Theinteractions

of

electron

and atom

are

various.As

the

energy

of

the

electrons

bombarding

the increases,

the

high-energy

limit

of

the

x-ray

spectrum

increases

correspondingly.2010/29/2021Radiatio–

Characteristic

rad.EmergenectronFreeelectron电子IncidentelectronPhoton,

X-ray,

a

vacancy

or

“hole”

is

left

in

thes

.An

electron

may

move

from

one

s to

another

to

fill

the

vacancy.Thismovement,termed

an

electrontransition(电子跃迁),This

transition

results

inthe

release

ofenergy–characteristic

X-ray.Characteristic

radiation

特征（标识）辐射CharacteristicphotonWhen

an

electron

is

removed

from

a

s2110/29/2021Characteristic

radiationK-characteristic

X-rayLcharacteristicX-rayOnly

K-char.

x-ray

is

useful

for

medical

appl.,

L,

M

will

be

filtered

for

low

energy.Only

while

eV>Wk,

characteristic

x-ray

would

be

generated.The

emitted

photon

may

be

described

as

a

K,

L,

or

Mcharacteristic

photon,

denoting

the

destination

ofthe

transition

electron.K-characteristic

x-ray

indicate

the

feature

of

element.KeV

WeV

WKFor

Tungsten,the

binding

energy

is

69.51keV,so

K-char.

tube

voltage?KW

hcRZ2V

1.36

102

Z

22210/29/2021Efficiencyof

X-rayTubeX-ray

Efficiency:The

efficiency

of

x-ray

production

is

the

ratio

ofenergyemerging

as

x

radiation

from

the

x-ray divided

by

the

energy

deposited

byelectrons

im ing

on

the

.Electrons

deposit

energy-

PowerdepositionRadiation

powerPd

IVrP

KIZV

2X-ray

production

is

a

very

inefficient

process,

even

inatomic

number.s

with

high2310/29/202110/29/202124X-rayFiltrationInherent

filtrationMost

x-rays

tubes

isabout

1mm

Al.External

filtrationThe

effect

of

theadded

aluminum

is

todecrease

the

total

number

of

photons

butincrease

the

average

energy

of

photons

inthe

beam.X-ray

“harder”Duel

energy

subtraction

research2510/29/2021Part

ⅢX-raytubefocalspot

andrated

parameterFocalspotof

an

x-ray

tube

ismounted

at

a

steep

angle

(6-17o)

with

respect

to

the

direction

of

im ing

electrons.With

the at

this

angle,

x

rays

appear

to

originate

within

a

focal

spot

muchsmaller

than

the

volume

ofthe absorbing

energy

from

the

im ing

electrons.For

most

x

ray

tubes,

the

size

of

the

focal

spot

is

not

constant.

Instead,

it

varieswith

both

the

tube

current

and

the

voltage

applied

to

the

x-ray

tube.Focal

spotApparent

FSThe

line-focus

principle(线焦点原理)To

reduce

the“apparent

size”

of

thefocal

spot,

the2610/29/2021Focal

spot

influences

image

qualityP

f

fdPS2710/29/2021Heel

effectHeel

effect

definitionFor s

mounted

at

a

small

angle,the

attenuation

is

greater

for

x

raysemerging

along

the

anode

side

of

thex-ray

beam

than

for

those

emergingalong

the

side

of

the

beam

nearestthe

cathode.

The

x-ray

intensitydecreases

from

the

cathode

to

theanode

side

of

the

beam.

Called

heeleffect(足跟效应).Two

reasonsElectron

energy

lossAttenuation

inHighLow2810/29/2021Rating

for

X-ray

tubeum

Energy

额定参数For

single

phase:

Number

of

heat

units

(HU)

=(Tube

voltage)

(Tube

current)

(Time)=

(kVp)

(mA)

(sec)For

three

phases:

Number

of

heat

units

(HU)

=1.35

(Tube

voltage)

(Tube

current)

(Time)

=

(kVp)

(mA)

(sec)Example:From

the

energy-rating

chart

at

right,

is

a

radiographictechnique

of

150

mA,

1

second

at

100

kVp

permissible?A:crackedbylack

ofrotation.

B:damaged

by

slowrotationandexcessive

loading.

C:damaged

byslowrotation.2910/29/20213010/29/2021Part

ⅣX-ray

radiationdoseRadiationX线穿越物体（如

）时：部分能量会留存在

中这些能量可能会引起

组织某些性质的改变由于电离产生离子对，引起辐射的电离损伤可能直接打破组织的分子键，导致分子损害-

…3110/29/2021Fluence,flux&

IntensityΦ

NAA

t

NThe

radiation

may

be

defined

as

the

motion

of

energy.photon

fluence（光子流量）photon

flux（光子通量）energy

flux（能量通量/射线强度）energy

fluence（能量流量）A

E

=

NEAt3210/29/2021I

E

=

NEFluence,Flux

&Intensity1eV=1/6.24e18

JW/m2Example

6-1An

abdominal

radiograph

uses

1013

photons

to

expose

a

film

with

an

area

of

0.15m2

(1.5

×

10−1

m2

or

1500

cm2)

during

an

exposure

time

of

0.1

second.

Allphotons

have

an

energy

of

40

keV.

Find

the

photon

fluence

,

the

photon

flux

φ,the

energy

fluence

,

and

the

intensity

I.3310/29/2021Radiation

exposure &absorbed

doseRadiation

exposure

unit（照射量）

-Roentgen

(R):而言就是Radiation

dose

unit（吸收剂量）

-Gray

(Gy):The

gray

(Gy)

is

a

unit

of

absorbed

dose.

定义为1kg物质（对组织）吸收1焦耳能量时的辐射.1

Gy

=

1

J/kg=100

RadExample

6-5)If

a

dose

of

0.05

Gy

(5

centigray

[cGy])

is

delivered

uniformly

to

the

uterus(during

a

diagnostic

x-ray

examination,

how

much

energy

is

absorbed

byeachgram

of

theuterus?One

is

release

energy

(or

dose),

the

other

is

absorbed.5e-5J3410/29/2021Radiation

exposure &absorbed

doseRelative

biologic

effectiveness

(RBE)（相对生物效能）In

order

to

compare

the

dose

relationship

on

certain

biologicaleffectiveness

caused

by

different

radiation

sources.RBE

DrefDtestIs

the

required

X-ray(250keV)

absorption

dose

to

cause

a

certain

biologicaleffectivenessDrefDtestIs

the

required

absorption

dose

from

other

radiation

source

to

cause

thesame

biological

effectivenessRBE

dose

SI

unit

(Sievert,

Sv)RBE剂量=吸收剂量*RBE3510/29/2021Equivalent

dose

&

effective

doseEquivalent

dose(当量剂量)The

term

equivalent

dose

was

adopted

to

refer

to

thepro