Medical Physics secondaty level slides

Incident Wave I0

transmitted Wave I2 transmitted

Wave I3

Reflected waves

Reflected wave R1 Reflected wave R2

Antenuation causedBy scattering, diffraction,Absorption/dissipated

I23

I12= I1 x e- µ1x1 I23= I2 x e- µ 2x2

Thickness x1,Acoustic Impedance Z1

Attenuation Coff µ1

Thickness x2,Acoustic Impedance Z2

Attenuation Coff µ2

R0

Attenuated R1Attenuated R2

t

Intensity

T1=2 X1/(V1)

T1=2 X1/(V1) + 2 X2/(V2)

Reflection of R1 at medium 0 and 1 boundary

Reflection of R2 at medium 0 and 1 boundary

I12

R0

R0 = α1 = (Z1-Z0)2

I0 (Z1+Z0)2 (simple ratio, dimension, remember the square)

0<= α<=1; If Z1=1/2 Z0=> α=1/9

I1 = (1-α1 ) I0= 1- (Z1-Z0)2

(Z1+Z0)2

= I0-R0

I0

I2 = (1-α2 ) I12= 1- (Z2-Z1)2

(Z2+Z1)2

= I12-R1

= I1 x e- µ1x1 1- (Z2-Z1)2

(Z2+Z1)2

I12

Ultrasound –Ultrasound –Reflection/echo;Reflection/echo; attenuation attenuationAcoustic impedanceAcoustic impedance

Key Physical theory:Reflection at bounduaryAttenuation and Acoustic impedance

f↑ resolution improves; Power ↑ but µ ↑ penetration depth ↓

Impedance ≠ attentuation Z=ρC (unit Rayl, kgm-2s-1)

(attenutaion lead to energy loss (as heat etc)Difference in impedance leads to reflection of sound energy)

µ x X is pure number

X in cm=> µ in 1/cmI (intensity) in Watt/m2

transmitted Wave I1

Key Word: TransducerPiezoelectric effect

A-Scan – ultrasound transducer + signal generator+ oscilloscopePrinciple – pulse echo (due to reflection) distance measurement

Show objects of different range and depthsElectirc pulse generates a ultrasound pulse by transducer and measure the time lag of echo (echo deforms transducer and generate electric signal)

B- ScanRepresent reflected intensity (spikes) AS brightness.Spots of different brightness proportional to Intensity of echo. By sweeping the transducer through an arc or use an array of transduceer a series of strips of scan over an area forms 2 D image

Acoustic shadow

Highly reflecting Gall stone

f↑ resolution improves; Power ↑ but µ ↑ penetration depth ↓

Incident X Ray I0

I1= I0 x e- µ1x1

Thickness x1, Attenuation Coff µ1

X-Ray; CT Scan Anttenuation causedBy scattering, diffraction,Absorption; for X Ray by ionising the tissue the X-Ray photon is absorbed-attenuated

Bone/

Air/low anttenuationorgan

Arteries/Intestine with artificial Contrast media

I0

ab

I1≈ I0 x e- µ1(a+b)

X-Ray partiallystopped by Bone/tissue with high attenuation

Metal MetalX-Ray stopped by Contrast media/Metal

E1Highest exposure

E2- Highexposure

E3 Lowexposure

E4 Mediumexposure

Exposure E1>E2>E4>E3

Transmitted beam vs reflected beam in US

RadionuclidesCharacteristics of Radionuclides used:-Non-toxic-produce Gamma ray only, no alpha and beta (range, damage)-Physical half life a few hours: long enough to allow imaging, not to long to cause lasting problem to body and risk of disposal-Decay to stable nuclide- Availability (technicium cow) and cost Technetium 99m decay into stable technetium (ie, not radiating), 6 hour half life), excreted in urine emits gamma only.

Problem of radiation hazard after excreted by body-still raidating for a few years in environment

Technetium -99m

Safety precaution:Limit the number of examinationKeep distance from radio active source except during exmainationShielding of source(Cow made of lead container)Containment- room with –ve pressure, proper disposal,