Post on 16-Jan-2016
description
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,