Introduction to Medical Engineering (Medical Imaging) Suetens 6 Ultrasound Imaging Ho Kyung Kim Pusan National University Sound Sonic: 20 Hz 20 khz (audible frequency) Subsonic (<) and ultrasonic (>) Ultrasound Acoustic wave Reflects, diffracts, refracts, attenuates, disperses, and scatters when it propagates through matter Ultrasound imaging Estimates the tissue position by measuring the travel time of ultrasound that reflects at the interface between different tissues (with the known acoustic wave velocity) Noninvasive, inexpensive, portable Excellent temporal resolution Being applied to nondestructive testing (NDT) and sound navigation ranging (SONAR) Not only to visualize morphology or anatomy but also to visualize function by means of blood and myocardial velocities (velocity imaging Doppler imaging) 2
Ultrasonic waves Progressive longitudinal compression waves (opp. transverse waves) Generated and detected by a piezoelectric crystal (which deforms under the influence of an electric field and, vice versa, induces an electric field over the crystal after deformation) 3 Acoustic impedance = = (acoustic pressure/particle velocity or mass density wave velocity) Substance (m/s) (10 6 kg/m 2 s) Air (25 C) Fat Water (25 C) Soft tissue Liver Blood (37 C) Bone Aluminum 346 1450 1493 1540 1550 1570 4000 6320 0.000410 1.38 1.48 1.63 1.64 1.67 3.8 7.4 17.0 4
Wave equation =0(linear wave eq.) General solution:, = + (+) e.g.,, = sin = sin ( ) Any waveform propagates through a medium without changing its shape Acoustic intensity (in units W/m 2 ):! = " d e.g.,! = $ " d= % $ The above is only valid when is only an infinitesimal disturbance of the static pressure. If not, wave propagation is associated with distortion of the waveform 5 Interference Constructive or destructive interference between waves Depending on the difference in traveled distance w.r.t. the wavelength Diffraction: a complex interference pattern resulted from an infinite number of coherent sources (i.e., sources with the same frequency and a constant phase shift) 6
Attenuation Loss of acoustic energy of the ultrasonic wave during propagation (e.g., conversion into heat in tissues because of viscosity) &(,')=( )*+ ( )* %-. + / =ln + per cm or nepers(np) per cm attenuation coefficient / in units of db/cm when //20log( or //8.6859( 20log + in db) / in Np/(cm MHz) or db/(cm MHz) typically <=1 Substance / (db/(cm MHz)) Nonlinearity (>/) Lung Bone Kidney Liver Brain Fat Blood Water Spleen Muscle 41 20 1.0 0.94 0.85 0.63 0.18 0.0022 7.2 7.5 11.0 6.2 5.0 7.8 6.5 7 Reflection and refraction Not only does the direction of propagation at the interface between two media change, but also the amplitude of the waves Snell's law for direction:?@ab C D =?@AB E D =?@AB F Note that transmitted wave is refracted wave cosh = 1 I ID sinh J Complex number if > & H J >sin ) (incident& reflected waves are out of phase) Amplitudes: L =M 1 Transmission coefficient: M N F N C = $ OP?B C $ OP?B C Q$ D OP?B F Reflection coefficient: L N E N C = $ OP?B C )$ D OP?B F $ OP?B C Q$ D OP?B F Reflection is large if and differ strongly (e.g., tissue/bone, air/tissue) 8
9 Scattering 10
Doppler effect If an acoustic source moves relative to an observer, the frequencies of the observed and transmitted waves are different the Doppler effect Doppler frequency: R = S = T U WOP?B QT U e.g., If a scatterermoves away from the transducer with a velocity of 0.5 m/s and the pulse frequency 2.5 MHz, the Doppler shift is approximately -1.6 khz. 11 Generation & detection Transducer Transmitter (when generating) Detector (when detecting) Consists of Piezoelectric crystal Usually polymeric materials, such as PZT (lead zirconatetitanate) and PVDF (polyvinylidene fluoride) The amplitude of the vibration, driven with a sinusoidal electric signal, is maximal when the thickness of the crystal is exactly half the wavelength of the induced wave (called resonance with the fundamental resonance frequency) Backing layer To reflect the reflected energy from the crystal back into the crystal Matching layer To reduce the impedance between the crystal and tissues 12
Gray-scale imaging A-mode (amplitude) imaging Based on the pulse-echo principle Measurements of the reflected waves as a function of time X= Detected signal ~MHz range ( called RF signal) M-mode (motion) imaging Repeated A-mode measurements for a moving object 13 B-mode (brightness) imaging Repeated A-mode measurements by translating or tilting the transducer 14
Image reconstruction For the acquired RF data, perform filtering, envelope detection, attenuation correction, logcompression, and scan conversion Filtering To remove high-freq. noise To remove the transmitted low-freq. band signal for the second harmonic imaging 15 Envelope detection To remove high-freq. information by means of a quadrature filter of a Hilbert transformation Attenuation correction To compensate the attenuation of reflected waves with depth using attenuation models Also called time gain compensation because of the linear relationship between time and depth 16
Log-compression To reduce the large dynamic range between the specular and scatter (appeared as speckle patterns) reflections by using a gray level transformation (using a logarithmic function) Scan conversion To convert polar-grid data (obtained from the tilting method) into a rectangular-grid data (obtained from the translating method) Also called sector reconstruction 17 Ex) A typical echographincludes 120 image lines and each line in the image corresponds to a depth of 20 cm. Assuming =1540m/s, what is the acquisition time of an image? The travel distance to and from the transducer is 40 cm. Therefore, the acquisition time of each line is 267 µs, and then that of the image is about 32 ms. Because the reconstruction time is negligible, the temporal resolution is 30 Hz (i.e., 30 images per second) 18
Doppler imaging To visualize velocities of moving tissues 3 different data acquisition methods 1) Continuous wave (CW) Doppler Two crystal for transmitting continuous sinusoidal wave and detecting in the same transducer Only exception to the pulse-echo principle No spatial (i.e., depth) information 2) Pulsed wave (PW) Doppler Transmitting pulsed waves at a constant pulse repetition frequency (PRF) M-mode acquisition but only one sample per each line at a fixed time (i.e., range gate), resulting in one specific spatial position 3) Color flow (CF) imaging B-mode acquisition but, for each image line, several pulses (3 7) instead of one are transmitted 2D anatomical gray scale image onto where the color velocity information is superimposed 19 CW Doppler Calculate the velocity of a scattering object (e.g., cardiac and blood velocities) using the relationship between R and ^ Encode the spectral amplitude obtained from the Fourier transform of the timelysegmented received signal into a gray value spectrogram or sonogram Trade-off between the velocity resolution and temporal resolution 20
PW Doppler Not make use of the Doppler principle Instead, S = and take only one sample of each of the received pulses at a fixed range gate Then, the received signals are becomes samples of a slowly time-varying sinusoidal function with frequency R = T U 21 CF imaging Similar to the PW Doppler, but CF imaging calculates the phase shift between two subsequent received pulses instead of calculating ^ from samples of a signal with R Alternatively, the phase shift can be calculated by cross-correlation of the signals received from two pulses 22
Spatial resolution Distinguished to be the axial, lateral, and elevationresolution according to the direction of wave propagation Axial resolution Determined by the duration Mof the transmitted pulse = e.g., =~0.5mm for a typical 2.5 MHz 23 Lateral and elevation resolution Typically, a few mm in the focal region 10 worse than the axial resolution Determined by the width of the ultrasonic beam (roughly ~the size & shape of the transducer) e.g., planar vs. concave crystals main lobe side lobe 24
Noise & contrast Noise Due to scatter reflections, called speckle noise However, the speckle pattern enables the user to distinguish different tissues from each other Contrast Echogenic structures with bright reflections: calcifications or tissue interfaces Hypogenic structures with weak reflections: blood The degree of perceptibilityof tissues is not only defined by the contrast, but also by the difference in speckle pattern or texture 25 Artifacts Side lobesin the lateral pressure profile produced by a focused transducer introduce information from another direction in the received signal Part of energy of a reflected wave can be reflected again by the transducer surface, and propagates through the tissues. These higher order reflections, called reverberations, can give rise to phantom patterns at a distance of multiples of the depth. 26
Aliasing Common artifacts in pulsed Doppler methods (i.e., PW and CF) due to undersampling Aliasing restricts the exact measurement of the scatterer velocity ^ < `a % - b 27 Ultrasound scanner The smalland mobileultrasound scanner mainly consists of a transducer connected to a signal processing box, which displays the reconstructed images on a monitor in real time 28
1D array transducer To reduce mechanical motion of a transducer for scanning Collection of many small identical crystals Linear-array transducer Electronically represents the mechanical translation motion by firing its elements sequentially Used for vascular imaging and obstetrics (large acoustic window) Phased-array transducer Electronically represents the mechanical tilting motion by tuning the phases of the waves sent by the different crystals and thus steering the direction of propagation of the wave Used for cardiology (small acoustic window between the ribs) 29 3D imaging 30
Specific transducers 31 Gray scale imaging Echography is useful if the ultrasonic waves are able to reach the tissues under examination and if the specular or scatter reflections, or both, are high enough to be perceived in the image. Consequently, it is limited to soft tissues, fluids, and small calcifications that are preferably close to the patient's body surface and not hidden by bony structures. Head Limited to the brain of a newborn (neonate) 32
33 Neck Thyroid, salivary glands, and lymph nodes Thorax Thyroid, salivary glands, and lymph nodes 34
Breast Mostly combined with mammography for differential diagnosis Fetus, uterus, and placenta 35 Abdomen Spleen, pancreas, and liver 36
Urogenital tract Kidney, bladder, prostate, testicles, uterus, vagina, and ovaries Transrectal and transvaginal transducers 37 Vascular system Diagnosis of dilations (aneurysm) and obstructions (stenosis, thrombosis) 38
Musculoskeletal system Diagnosis of tears, calcifications, and acute inflammations with edema in the muscles, tendinous, and capsular structures 39 Heart Ventricles and atria, aorta, valves, and myocardium 40
Doppler imaging Flow imaging Measurements of flow in the blood vessels and the heart Strain imaging Measurements of the local deformation of the tissue 41 Contrast echography Ultrasound imaging using a solution with microscopic air bubbles (typ. diameter 4 µm) injected into the blood circulation, for example, to visualize fluid cavities and for the assessment of organ perfusion 42
Biological effects and safety Tissue heating Tissue damage due to heat converted from ultrasonic energy Monitor the thermal index based on the transmitted power not to exceed a certain threshold Heating can be used for ultrasound surgery to burn malignant tissue Cavitation Tissue damage due to the collapse of bubbles that are formed in areas of low local density resulting from a negative pressure Monitor the mechanical index based on the peak negative pressure not to exceed a certain threshold Cavitation is the basis for lithogripsers, which destroy kidney or bladder stones by means of highpressure ultrasound 43