Recent advances in USG

Presented by:

Nisha karna

M.Sc. MIT 1^st year

Roll no: 25

Introduction

• Ultrasonography in the last few decades has undergone massive transformation and today occupies a crucial role in practice of most of the domains of medicine.
• Advances in ultrasound technology include enhanced spatial, vascular and contrast resolution, besides encompassing various therapeutic options.
• New transducers and emerging imaging paradigms allow real time acquisition of large field of views and even three dimensional volumetric data.

Some recent advances in USG

• Tissue harmonic imaging
• Compound imaging
• Extended Field of View
• Coded pulse excitation
• Adaptive image processing
• Elastography
• Contrast enhanced US
• Electron section focusing
• Fusion imaging
• Some advancement in transducer

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Tissue harmonic imaging

• Tissue harmonic imaging (THI) is a grayscale ultrasound mode that provides images of higher quality than conventional sonography by using information from harmonics
• In conventional grayscale sonography, the same frequency spectrum that is transmitted into the patient is subsequently received to produce the sonographic image.
• In THI, however, higher harmonic frequencies (multiples) generated by the propagation of the ultrasound beam through tissues are used to produce the image.
• Currently, only the second harmonic, or twice the fundamental frequency, is used for imaging.
• The final image is formed by the harmonic frequency bandwidth in the received signal after eliminating the transmitted frequency by frequency filtering, pulse inversion/phase cancellation or coded harmonics.

Tissue harmonic imaging

• The higher harmonic can also be used but extremely wide bandwidth transducers would be required and the intensity decreases as the order of the harmonics increases also the higher frequency harmonics are attenuated more.
• Harmonic imaging is especially useful in obese patients because of reduction in deleterious effects of body wall.
• Intensity of harmonic waves generated depends on the non-linearity coefficient of the tissue insonated.

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Tissue harmonic imaging

• Tissue harmonic imaging has a huge potential for improving image quality because it gives improved lateral resolution, reduced side lobe artifacts and improved signal to noise ratio
• Although better demonstration of cystic structures by THI has been emphasized, authors have now observed that tissue harmonic imaging provides additional information in both solid and cystic lesions.

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Tissue harmonic imaging

Tissue harmonic imaging

Tissue harmonic imaging

Tissue harmonic imaging

• Another harmonic technique, known as 1.5 harmonic imaging has also been described.
• This detects and visualizes signals that are a factor of 1.5 times higher than the fundamental center frequency of the transducer and are intermediate between the fundamental and the second harmonic frequency spectrum.
• Obvious advantage is that this frequency range is nearly free of tissue echoes and only contains microbubbles echoes.
• Hence, it is featured with contrast improvement between tissue and microbubbles of 20dB or more compared to the second harmonic technique.

Compound imaging

• Compound imaging is the ability to aquire multiple frames from different-frequencies, i.e. frequency compound imaging or from different angles, i.e. spatial compound imaging
• Frequency compounding is the combination of multiple images detected from different frequency bands into a single image.
• This reduces noise and speckle as the appearance of speckle varies with frequency.
• This is possible with the use of broad band width transducers.

Spatial compound imaging

• Spatial compound imaging is a speckle reducing ultrasound technique, which uses electronic steering of ultrasound beams from a transducer array to obtain overlapping scans of a target object from different angles
• Resulting echoes from these multiple acquisitions are then averaged to produce a single compound image of improved quality due to reduction in image speckle.
• Compared to conventional B-mode imaging, in compound imaging, more time is required for acquisition of data and frame rate is also reduced.
• Spatial compounding is also useful to image specular reflectors

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Spatial compound imaging

• Spatial compound images show reduced levels of speckle, noise, clutter and refractive shadows and improve contrast and margin definition
• Enhancement and shadowing artifacts may be reduced
• In imaging of breast, peripheral vessels, and musculoskeletal system.
• It can also be combined with other ultrasound applications, e.g. harmonic imaging.

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Spatial compound imaging

Spatial compound imaging

Spatial compound imaging

Extended field of view�

• Extended field of view imaging, also called panoramic imaging allows sonologists to visualize large anatomic regions in a single image.
• It can be performed on superficial structures with a linear array transducer or abdominal structures using a curvilinear probe
• Transducer is initially moved laterally across the anatomic area of interest and multiple images are acquired from many transducer positions.
• Images are registered with respect to each other.
• The registered data is subsequently combined to form one complete large field of view image.

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Extended field of view

Coded pulse excitation

• Higher frequency US pulses produce images with better spatial resolution, however, increased attenuation with increasing frequency limits evaluation of deep structures with high frequencies
• Coded pulse excitation is a means of overcoming this limitation providing good penetration at higher frequencies necessary for high spatial resolution.
• Uses longer ultrasound pulses instead of the routinely used short ultrasound pulses.
• The coded pulses are produced with a very characteristic shape and the returning echoes also have a similar shape, which makes their identification from background noise easier

Coded pulse excitation

• The returning echoes are then processed using a pulse compression technique in which the location of the long characteristic pulse shapes are identified.
• Produces an image with good echo signal and good spatial resolution at greater depths.

Adaptive image processing

• An adaptive image processing algorithm is one that able to recognize the difference between real targets and artifacts, and to modify its processing accordingly.
• Multiresolution adaptive image processing algorithms to smooth speckle and enhance structural edges are now available (e.g. XRES Philips).
• It involves an analysis phase (in which artifacts and targets are identified) and an enhancement phase (in which artifacts are suppressed and targets enhanced)
• The analysis phase takes into account multiple characteristics of the target, such as local statistical properties, textural and structural properties.
• The textural and structural information in particular is vital for identifying the strength and orientation of interfaces and thus allowing directional filtering of these targets in the enhancement phase

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Elastography

• Elastography is a noninvasive technique of imaging stiffness or elasticity of tissues by measuring movement or deformation of tissue in response to a small applied pressure.

Stress

• Stress is defined as force per unit area. Unit of stress is Pascal or pounds per square inch (psi).(Pascal = Newton/m2 ).
• Stress can be due to compression, which acts perpendicular to a surface and causes shortening of an object. Shear stress acts parallel to a surface and causes deformation
• Shear stress acts parallel to a surface and causes deformation.

Elastography

• In elastography, stress can be applied exogenously-by transducer compression, vibrators or acoustic radiation force.
• Endogenous motion of vessels, cardiac or respiratory motion can also be utilized.
• Although endogenous sources overcome the shortcomings of exogenous source like attenuation (e.g. due to obesity or ascities), endogenous stress is difficult to quantify

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Elastography

Strain

• When subjected to stress an object tends to undergo deformation of its original size and shape; the amount of deformation is known as strain.
• Longitudinal strain, like compression causes change in length of an object.
• Shear strain causes changes in angles of an object. Strain is unitless, expressed as change in length per unit length of the object.
• When compression is applied, lesions nearer the applied force undergo more displacement than objects lying in a deeper plane

Elastography

Elasticity

• The property of materials to return back to its original form after stress is removed is known as elasticity

Viscosity

• Viscosity is the measure of resistance of a fluid when it undergoes shear stress or tensile (compression or stretching)

Elastography

Viscoelasticity

• This is the property of materials to exhibit both viscous and elastic properties

Poroelasticity

• A material in which a solid matrix is permeated by an interconnecting network of fluid filled pores.
• Tissue may be viscoelastic, poroelastic, anisotropic

Elastography

• Young’s Modulus (E) is the ratio of stress over strain and has the same unit as stress. Young’s Modulus measures the tissue’s resistance to compression.
• Poisson’s Ratio: When exposed to stress, tissue may contract in one dimension (like width) while its length increases. Poisson’s ratio = lateral contraction per unit breadth/ longitudinal extension per unit length.
• Poisson’s ratio for normal soft tissue is 0.5.
• Shear modulus: Shear modulus or modulus of rigidity (G) is a ratio between shear stress and shear strain.

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Elastography

DEPICTION OF ELASTOGRAMS:

• Elastograms are generally viewed simultaneously with a sonogram to identify area of abnormality.
• Can be done with either Gray Scale depiction or a semi-transparent color overlay of the elastogram over the sonogram.
• On Gray Scale elastograms, stiffer lesions are darker and appear to increase in size when compared to sonograms.
• On color overlay images, blue and green depicts stiff areas and red to yellow denotes soft areas.

Elastography

DIFFERENT ELASTOGRAPHY TECHNIQUES

1. The method used for tissue excitement (either mechanical or ultrasonic force)
2. By the response of tissue to compression, i.e. static or quasistatic, where a single compression is applied or dynamic system in which response to rapid compression or vibration is measured.

Compression elastography

• Compression elastography is calculating a strain profile in a direction perpendicular to the tissue surface in response to an externally applied force.
• This technique is most widely used in different ultrasound systems to evaluate the elastic properties of the tissues by analyzing the radiofrequency pulses generated by a structure in response to external compression.
• RF waveform before and after compression are windowed and the signals in the same segment are compared to calculate the displacement.

Compression elastography

• A simple approach to extract elasticity information from soft tissue involves acquiring maps of the tissues before and after the compression
• The amount of shift in the signal equals the amount of tissue displacement at the point in the image frame
• Three methods have been introduced for measuring tissue strain at elastography are the spatial correlation method, phase shift tracking method and combined autocorrelation method.

Compression elastography

• The spatial correlation method is 2D pattern matching algorithm to search for the position that maximizes the cross correlation between ROIs selected from two images.
• The phase shift tracking method is based on autocorrelation method and can be used to rapidly and precisely to determine longitudinal tissue motion because of phase domain processing.
• This method fails when used for large displacement and it poorly compensates for lateral movement known as lateral slip

Compression elastography

• To overcome this problem a method known as combined autocorrelation method is employed, which enables rapid and accurate detection of longitudinal displacement by using phase domain processing without aliasing
• The deformation measurement is mapped on elastogram on which stiffer areas are depicted as dark and more elastic area are lighter.

Compression elastography

Limitations

• The amount of tissue displacement and the rate of change in displacement vary with the amount of compression applied.
• It does not quantify the intrinsic elastic property of the given tissue which makes it operator dependent.
• It is a qualitative imaging of relative stiffness so the actual strain value cannot be compared with the next imaging
• It is suitable for the detection and evaluation of the small focal lesion and not sensitive to the diffuse disease process that produces same stiffness all over in one image.

Acoustic Radiation Force Impulse

• Acoustic radiation force impulse (ARFI) imaging is a technique where short duration acoustic forces known as pushing pulses are used to cause tissue displacements.
• No external or physiological (pulsation or respiration) compression is needed. The cessation of force causes tissue to return to its original position.
• Pushing pulses can be applied by the ultrasound transducer array (frequency of 2-7 MHz) to a volume of 2 mm3 for 1 ms per pulse resulting in a displacement of 10 to 20 m

Acoustic Radiation Force Impulse

• Ultrasound pulses track these displacements by locating change in the peak along multiple tracking lines.
• The excitation can be performed in a sequential fashion by translating the tracking line along the tissue to assess the response.
• Parallel acquisition of push pulses and tracking displacements can also be done

Acoustic Radiation Force Impulse

• Tissue recoil also generates shear waves, which propagate away from the focal point of excitation.
• The speed of the shear waves is proportional to the tissue stiffness. Shear modulus can be calculated from the shear wave velocity
• Mapping shear wave velocity (cm/sec or m/sec) at multiple lateral points from the region of excitation can generate quantitative measurements of tissue stiffness.

Acoustic Radiation Force Impulse

Advantages

• The ARFI images are found to be more homogeneous and have better contrast than surface displacement (compression) elastography.
• Deeper tissue, not accessible by superficial external compression elastography can be evaluated.

Disadvantages

• Physiological (respiration, pulsation) and transducer motion can degrade image quality as 1 to 3 ms is required per tracking line pair
• Tissues at a depth of more than 10 cm cannot be accurately assessed due to attenuation of the radiation force at greater depths.

Shear Wave Imaging

• Shear waves are induced remotely within tissue when an impulsive acoustic radiation force of a focused ultrasound beam produced by the transducer interacts with tissue.
• Shear waves propagate perpendicular to the axial displacement caused by the ultrasound pulse and attenuate about 10000 times more rapidly than compression waves
• Velocity of shear waves (in cms–1) can be measured and used to evaluate tissue stiffness by calculating the elastic Young’s modulus
• This technique results in both qualitative color coded elastogram and also quantitative maps either of elasticity

Shear Wave Imaging

Advantages

• Lack of tissue compression makes it a more objective measurement, the direct assessment of elasticity and the quantitative measurements are provided.

Disadvantages

• Assessment of superficial structures may be difficult, as a certain depth of ultrasound penetration is needed for shear waves to be produced.

Shear Wave Imaging

Advances in Shear Wave Imaging

1. Spatially modulated ultrasound radiation force (SMURF):
1. In this technique, a linear array transducer is used to acquire a reference scan at a specified position.
2. Two pushing pulses are then transmitted and focused at the same depth laterally from the original position which is followed by a series of scan lines, from which the induced shear wave peaks are estimated.
3. This allows fast and accurate estimation of shear modulus with improved resolution.1

Shear Wave Imaging

Supersonic shear wave imaging:

• In supersonic shear wave imaging, the focus of the radiation force from one location is changed to different depths (typically five) along the beam axis.
• Shear waves are created at multiple locations and these interfere constructively to create a conical shear wave
• Imaging the shear wave propagation requires an ultrafast scanner capable of 5000 frames per second
• Shear wave propagation is measured and wave speeds are assessed to give viscoelastic moduli

Shear Wave Imaging

Axial shear strain imaging:

• Malignant lesions tend to be more tightly bound to surrounding tissue than benign lesions.
• Axial shear strain images how tightly the lesion is fixed to the surrounding tissue
• Loosely bound lesions have a thin band of color at the boundary whereas malignant lesions have a much thicker band which is much easier to interpret than elastography

Shear Wave Imaging

Shear Dispersion Ultrasound Vibrometry

• This involves creating a shear wave by an external actuator or by acoustic radiation force.
• Multiple pushing pulses or excitation pulses are transmitted at a particular frequency and motion stimulated at harmonic frequencies is detected by ultrasound.
• Shear wave speed dispersion is measured from the data generated at several frequencies.
• Dispersion of shear wave gives a measurement of the viscoelastic properties of tissue

Mechanical Imaging

• Stress patterns of internal structures of tissue are measured by compressing the tissue by an ultrasound probe.
• Pressure sensors mounted on the contact surface of the probe detects the temporal and spatial changes in stress pattern, thus providing information about the different elastic properties (viscosity and porosity)

Applications of elastography

• Breast

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Elastography image of fibroadenoma showing elastographic score 2 with few small blue areas of no strain in the periphery. B-mode image shows well-defined ovoid hypoechoic nodule with posterior acoustic enhancement

Elastographic image of IDC breast shows elastographic score 5 with blue area of no strain in the lesion and in the peripheral echogenic halo

Applications of elastography

• Liver:
• Diffuse liver diseases: Fibrosis, Cirrhosis, Acute Hepatitis, Nonalcoholic Fatty Liver Disease
• Elastography (by ultrasound or magnetic resonance) is a noninvasive modality for assessment of fibrosis and degree of fibrosis.

Techniques for liver stiffness measurement

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• Transient elastography- FibroscanTM apparatus (Echosens, Paris, France)—is a non-imaging modality which has been widely used for liver stiffness measurement
• It uses a single cycle of low amplitude low frequency (50 Hz) vibration to induce a shear wave.
• The velocity of the shear wave is faster in dense fibrotic tissue.
• Transient elastography probe has a 5 MHz ultrasound transducer probe mounted on the axis of a piston, which acts as a vibrator

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Image of the Fibroscan device (Echosens, Paris) for transient elastography. (A) The probe has a piston (arrow), which is placed in the right intercostal space.

Techniques for liver stiffness measurement

Applications of elastography

Prostate

• Prostate gland was one of the earliest organs for which elastography was proposed
• Real time endorectal elastography shows the sensitivity 68 percent with accuracy of about 76 percent.
• Prostate cancers have a higher elastic modulus than that of surrounding normal prostate tissue.

Elastography image is showing hard nodule in the prostate in the right peripheral zone in blue color. Same nodule appeared hyperechoic on B-mode ultrasound image

Endoscopic Ultrasonography

• Endoscopic ultrasonography (EUS) is one of the most recent advances in gastrointestinal endoscopy.
• The EUS with real time tissue elastography can be more useful than EUS with only a B-mode imaging ability.
• Real-time EUS elastography can be performed with the conventional EUS probes without any need for additional equipment that induces vibration or pressure.
• The main pitfall of EUS elastography is the inability to control tissue compression by the EUS transducer

3D and 4D Ultrasound

• 3D ultrasonography or volume sonography is the imaging technology which involves acquiring a large number of data sets of 2D images from patient.
• After acquisition, this volumetric data can be qualitatively and quantitatively assessed with the use of many analysis tools such as surface and volume rendering, multiplanar imaging and volume calculation techniques, etc
• If the 3D ultrasound is acquired and displayed over time, it is termed as 4D ultrasound, live 3D ultrasound or real time 3D ultrasound
• Currently there are two commonly used techniques to acquire 3D volumetric data - free hand technique and automated technique.

3D and 4D Ultrasound

• In the free hand technique, the examiner requires to manually move the probe within the region of interest.
• In the automated technique dedicated 3D probes (also called volume probes) have to be used.
• In the automated method, probe is held stationary and on activation the transducer elements within the probes automatically sweep through the ‘volume box’ which has been selected by the operator.
• The resultant images are digitally stored and can be ‘processed’ later in various display modes for analysis.

3D and 4D Ultrasound

(A) Transabdominal sonographic coronal 2D image showing bony spur in a case of diastemeomyelia, (B) is corresponding 3D image showing the bony spur

Applications of 3D and 4D ultrasound

Gynecology

• 1. For assessment of congenital anomalies of uterus
• 2. For evaluation of endometrial and uterine cavity
• 3. For preprocedure localization of fibroids for planning myomectomy

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Obstetrics

Chewing

First whinge

Sleepy

Smile

Contrast enhanced USG

• Contrast-enhanced ultrasound (CEUS) involves the administration of intravenous contrast agents consisting of microbubble/nanobubbles of gas.
• Contrast-enhanced ultrasound has the advantage over contrast-enhanced MRI and CT in patients with contraindications such as renal failure or iodinated contrast allergy.
• Contrast-enhanced ultrasound also allows for dynamic and repeat examinations.
• The main purpose of contrast enhanced US is to increase the signal intensity returning to the transducer
• The primary mechanism to increase the signal intensity in CEUSI is the scattering of US beam caused by microbubbles instead of blood

Contrast enhanced USG

 RCC in renal transplant. Grayscale imaging of the transplant kidney suggests the presence of a hypoechoic mass in the medial upper pole of the kidney mass (A, arrows). Administration of contrast more clearly defines the mass (B, arrow) which turned out to be renal cell carcinoma.

Fusion imaging

• An emerging technique in the field of abdominal imaging with translation possibilities to neuro radiology
• Involves co-registered display of live us with a refrence series from another modality such as CT, MRI or PET
• As the US exam is performed the fusion system continuously generates reformatted planes from the reference
• Series matching the oblique imaging planes of the US transducer
• The reformatted planes are displayed either as an overlay or side by side with the live US
• The display enhance interpretation of US by enabling a direct comparison with the refrence image from the same view angle.

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Fusion imaging

• Fusion imaging makes use of tracking system to localize US transducers and other devices relative to the patient
• Optical and electromagnetic systems are available, the latter being most commonly used.
• Various software tools are also used to bring the reference series into alignment with the tracking system for fusion display
• Tracking sensors are also incorporated into some interventional devices such as introducers and ablation needles, enabling the display of needle location as an overlay on live US image

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Fusion imaging

Fusion imaging and CEUS guided RFA of 1cm HCC in a pt with Hepatitis B virus

Electronic Section Focusing�

• Electronic focusing of the ultrasound pulse in the lateral direction by using transducer arrays provides great benefits compared with mechanical focusing by using curved piezoelectric elements or acoustic lenses
• Most transducers in current clinical use still employ mechanical focusing in the section thickness direction, resulting in highly variable section thickness which can cause great difficulty in accurately visualizing small structures.
• Electronic focusing is done with more than one dimensional single row of PZT elements ( referred to as  1.25D, 1.5D, 1.75D, or full 2D (two-dimensional) arrays, depending on the number of element rows and the level of independence with which the individual rows may be excited 
•  The greater the number of rows, the greater the ability for full electronic focusing. Full 2D arrays will be capable of providing multiple, user-selectable elevational focusing and more uniform section profiles

Electronic Section Focusing�

US images of a phantom show much improved demonstration of small anechoic spheres in the image obtained with electronic focusing in the section thickness direction (b) than in the conventional B-mode image (a). (Courtesy of GE Medical Systems, Waukesha, Wis.)

Miniaturization

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• Modern full-size US scanners are relatively portable and inexpensive, especially compared with imaging units for modalities such as MR imaging and CT.
• The general trend toward hardware miniaturization and the use of dedicated integrated circuitry is making possible even smaller and less expensive US scanners

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Wireless transducer

• They feature proprietary ultra-wideband wireless radio to help ensure images are transferred at high speeds with full data integrity and free from interfering with other medical equipment and all without cables
• These breakthrough cable free innovations help provide uninterrupted, hassle free, real time imaging that allows you to scan in the position that is comfortable for you and safe for you patient .

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Smartphone ultrasound

• Inexpensive , which lead to even wider use, especially in rural and third world areas
• Widespread use in emergency rooms, and in the surgical and medical wards as well as in office practice
• A great start for diagnosing all types of medical problems, as a screening device such as vascular problems, gallstones, kidney stones and other problems
• The model shown is a Mobisante which is worlds first smartphone based US imaging system, Mobi US SP1 imaging system

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References

• Arun kumar Gupta Diagnostic radiology recent advances and applied physics in Imaging
• Jerrold T Bushberg, The essentials physics of medical imaging third edition
• AAPM/RSNA Physics Tutorial for Residents: Topics in US
• Various websites

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Thank you