Ultrasound: Past and Present. Lecturer: Dr. John M Hudson, PhD

Transcription

1 Ultrasound: Past and Present Lecturer: Dr. John M Hudson, PhD

2 Disclosures 2 No conflicts of interest to declare

3 Course Outline 3 1. Survey of ultrasound physics & applications 2. (Sep 21) 3. (Sep 28) 4. (Oct 5) 5. (Oct 19) 6. (Oct 26) US fundamentals: Governing physics & principles US technology and image processing Blood flow and Doppler Nonlinearity: Harmonic and Contrast-Enhanced Ultrasound Bioeffects, Safety, and Review

4 Course Outline 4 (Nov 2) (TBD) Hands-on US lab, practice exam Final review, exam take-up

5 Course Facilitators 5 Chelsea Munding - cmunding@sri.utoronto.ca John Hudson - john.hudson@mail.utoronto.ca Coordinators Naomi Matsuura naomi.matsuura@utoronto.ca Eric Bartlett eric.bartlett@utoronto.ca

6 Today s Outline 6 i. Ultrasound: Versatility in diagnostics and therapeutics ii. The origins of medical ultrasound iii. iv. Progress toward modern ultrasound Introductory physics v. Comparison with other clinical imaging modalities vi. Preview of upcoming topics

7 The Versatility of Ultrasound in Medicine 7 Anatomy Function: Blood Flow Healthy Liver Colour Doppler

8 The Versatility of Ultrasound in Medicine 8 4D Surface Anatomy

9 The Versatility of Ultrasound in Medicine 9 Function: Tissue Perfusion Ultrasound Contrast Agents Microbubbles vibrate when exposed to ultrasound Used as blood pool tracers Contrast-enhanced ultrasound of the liver exposes microvascular structures in real time

10 The Versatility of Ultrasound in Medicine 10 Measuring Tissue Stiffness with Ultrasound Elastography Shear wave propagating in a phantom à Map of Tissue Stiffness 200 kpa ß Depth 1 cm The speed of the shear wave increases as it passes through a hard nodule

11 The Versatility of Ultrasound in Medicine 11 Safe enough for prenatal monitoring Fetal ultrasound scan (from Siemens.com) Powerful enough to destroy diseased tissue noninvasively Tissue fractionation in the prostate via histotripsy (from Hempel et al., J Urology 2011; 185: )

12 State of the Art: in the 1950/60s 12 The Pan Scanner University of Colorado 1957 First B-mode Ultrasound Scans

13 13 State of the Art: in the 1950/60s

14 Modern Clinical Ultrasound 14 Scanners Philips CX-50 Rivanna Accuro GE Logiq S8

15 Images courtesy aps.org, atlas-elektronik.com. A more thorough history: Founders of Ultrasound 15 Pierre and Jacques Curie, circa ~1880 Paul Langevin, French Physicist Inventor of the hydrophone and sonar The piezoelectric effect Sonar Crystal V + _ Crystal V +_

16 Progress in Technology 16 From: Diagnostic Ultrasound Imaging: Inside Out, Elsevier Academic Press.

17 Sound and Ultrasound 17 Courtesy of Riza Atav, The Use of New Technologies in Dyeing of Proteinous Fibers

18 Ultrasound propagation 18 Sound energy propagates as a longitudinal compressional wave

19 Basic definitions 19 Peak (high pressure) Trough (low pressure)

20 Basic definitions 20 Distance [metres] Wavelength (λ) [m]

21 Ultrasound propagation 21 Speed of sound (c): how quickly that wave propagates [m/s] Not the same as particle velocity! Note motion of red particle.

22 Basic definitions 22 Charting the pressure at a single point over time: period (T) [s] pressure time frequency ( f ) = 1/T [s -1 or Hz]

23 Basic definitions 23 pressure time f [Hz] = c/λ

24 Speed of sound Related to the strength of intermolecular interaction n Gases have weak intermolecular bonds, so the sound wave propagates slowly between molecules n Solids have strong intermolecular bonds, so sound propagates quickly 2. A function of compressibility and density

25 Wavelength example 25 What is the wavelength of a 5 MHz wave in the liver? (c liver = 1590 m/s) f = c / λ λ = c / f λ = 1590 m/s / Hz = m = mm

26 Pressure and Intensity 26 We often discuss the intensity of ultrasound Intensity = energy / square metre I = (pressure) 2 /(ρ*c), ρ= tissue density The decibel scale (db) is used to compare intensities: db = 10 log (I/I o ) A negative db value indicates that the intensity is lower, positive db indicates an increase Example: -3 db indicates that the sound intensity was reduced by half

27 Imaging with sound 27 We can find out how far away an object is by measuring the time it takes for an echo to return t=0 Ultrasound transducer creates a sound wave

28 Imaging with sound 28 We can find out how far away an object is by measuring the time it takes for an echo to return t=0 Ultrasound transducer creates a sound wave The sound wave travels through tissue at ~1540 m/s

29 Imaging with sound 29 We can find out how far away an object is by measuring the time it takes for an echo to return t=0 Ultrasound transducer creates a sound wave The sound wave travels through tissue at ~1540 m/s t=t 1 The sound wave is reflected by an object

30 Imaging with sound 30 We can find out how far away an object is by measuring the time it takes for an echo to return t=t 2 The reflected wave returns to the transducer t=t 1 The sound wave is reflected by the object Distance to object = 1540 m/s. t 2 / 2

31 Range finding with sound 31 If a dolphin detects an echo 0.01 second after it makes a chirp, how far away is the object? Clue 1: The speed of sound in water is 1480 m s -1 Clue 2: The speed of sound (rate) equals the distance travelled divided by the time taken Answer: distance = rate x time or distance = speed x time Put the numbers in: distance = 1480 x 0.01 = 14.8 m But this is the distance from the dolphin to the object and back, so the distance to the object is 7.4 m.

32 Range finding in tissue 32 Example of d=r t in tissue You send a pulse from an ultrasonic transducer into a patient at time 0 with a short-axis view of the bladder. You expect to see the proximal bladder wall at 5 mm depth. At what time will the signal return? Clue 1: The speed of sound in tissue is roughly 1540 m s -1 Clue 2: The time taken equals the distance travelled divided by the speed of sound (rate) Answer: time = distance/rate or time = distance/speed Put the numbers in: time =.005 / 1540 = 3.25x10-6 s = 3.25 µs But this is the time to the interface (one-way). We need to include the return time, so the total (round-trip) time is 6.5 µs.

33 Imaging with sound 33 Using this principle, we can also determine the distance between two objects t=t 1 Part of the wave is reflected by object #1. Part of the wave continues forward.

34 Imaging with sound 34 Using this principle, we can also determine the distance between two objects t=t 2 The transmitted wave encounters object 2 and is reflected

35 Imaging with sound 35 Using this principle, we can also determine the distance between two objects t=t 3 The first reflection returns to the transducer

36 Imaging with sound 36 Using this principle, we can also determine the distance between two objects t=t 4 The second reflection returns to the transducer

37 Imaging with sound 37 So at the transducer, we see: Amplitude t o : US pulse transmitted t 3 : 1 st reflection received t 4 : 2 nd reflection received Time (s)

38 Imaging with sound 38 Distance to 1 st object = t 3 / 2 Distance to 2 nd object = t 4 / 2 Amplitude t o : US pulse transmitted t 3 : 1 st reflection received t 4 : 2 nd reflection received Time (s)

39 Imaging with sound 39 But what if the two objects were close together? t=t 1 Reflection from object 1

40 Imaging with sound 40 But what if the two objects were close together? t=t 2 Reflection from object 2

41 Imaging with sound 41 Distance to 1 st object = t 3 / 2 Distance to 2 nd object = t 4 / 2 Sound is additive constructive/destructive interference. How can we resolve? What are the limits? Amplitude t o : US pulse transmitted Time (s)

42 Resolution 42 Medical ultrasound probes engineered to be optimal for specific applications (feature size, imaging window, depth) Imaging resolution depends on transducer design Lateral Axial (dir. of propagation) Elevational (into page) Image courtesy Jacaranda Physics 1, Wiley & Sons

43 Axial Resolution 43 As a simple approximation: Image formation uses envelope of returned signal (Lecture 3) Objects must be separated by at least half of the pulse length so that envelopes are distinguishable Axial Resolution (m) Pulse Length (m) / 2 Axial Resolution (m) n*λ / 2, where n = # cycles Axial Resolution (m) n*(c / f) / 2 Pulse length Env. 1 Env. 2 Amplitude Time (s)

44 Axial Resolution 44 What approximate resolution we can expect with a 2-cycle, 5 MHz pulse in tissue with a speed of sound of 1540 m/s? Clue 1: The length of the pulse is 2*wavelength of 5 MHz sinusoid Clue 2: The axial resolution is approx. half the pulse length Answer: (1) pulse length = 2*λ = 2*c/f (2) resolution pulse length/2 Put the numbers in: resolution [(2*1540)/(5x10 6 )] / 2= m = 0.3 mm

45 Lateral Resolution 45 As a simple approximation: Resolving laterally depends on how tightly we can focus energy with the probe analogous to a magnifying glass The bigger the probe, the better focusing we get As long as we re in the far field (not too close to the probe) Lateral Resolution (m) λ* Z / D where Z is the axial depth, D is the probe length D (meters) Image courtesy Jacaranda Physics 1, Wiley & Sons

46 Lateral Resolution 46 What approximate resolution we can expect with a 1 MHz probe imaging at 2 cm depth in tissue with a speed of sound of 1540 m/s if the probe is 3 cm in length? Clue 1: Lateral resolution depends on imaging depth, frequency, speed of sound in tissue, and transducer size Answer: resolution λ* Z / D where Z is the axial depth, D is the probe size resolution [(c / f) * Z] / D Put the numbers in: resolution [(1540/1x10 6 )*0.02] / 0.03 =.001 m = 1 mm

47 Elevational Resolution 47 As a simple approximation: Same concept as lateral resolution, but now talking about width of transducer rather than length The bigger the probe, the better focusing we get Elevational Resolution (m) λ* Z / H where Z is the axial depth, H is the probe width (into page) Image courtesy Jacaranda Physics 1, Wiley & Sons

48 48 Where does ultrasound fit within Diagnostic Imaging? From an ultrasound imaging book. But what about SNR, CTR, soft tissue contrast, volumetric information? From: Diagnostic Ultrasound Imaging: Inside Out, Elsevier Academic Press.

49 How is ultrasound different? 49 Significant operator dependence Direct contact with patient during imaging Diagnostic and non-diagnostic artifacts at operator level Knowledge of underlying physics aid interpretation Find a good sonographer! RSNA: US Artifacts by Feldman et al. (2009) Recommended and available on course website

50 How is ultrasound different? 50 Diagnostic and non-diagnostic artifacts at operator level Knowledge of underlying physics aid interpretation RSNA: US Artifacts by Feldman et al. (2009)

51 How is ultrasound different? 51 Diagnostic and non-diagnostic artifacts at operator level Knowledge of underlying physics aid interpretation Acoustic shadow caused by absorption and reflection of US by a kidney stone (arrow) RSNA: US Artifacts by Feldman et al. (2009)

52 How is ultrasound different? 52 Diagnostic and non-diagnostic artifacts at operator level Knowledge of underlying physics aid interpretation Acoustic shadow caused by absorption and reflection of US by a kidney stone RSNA: US Artifacts by Feldman et al. (2009)

53 Upcoming Topics 53 Ultrasound physics and image formation Speckle noise or not? Compounding Off Compounding On

54 From Doppler Ultrasound Evaluation of Renal Transplants by Piyasena et al., Applied Radiology 2010 Doppler Ultrasound 54

55 Contrast-Enhanced US 55 Courtesy J. Chomas, Siemens Ultrasound

56 Next Lecture 56 Sep 21