Ecography at high frequencies and sub-millimeter resolution

Elisa Prandini----------------------------

Medical Physics Exam23rd PhD Cycle

10 December 2009

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Ultrasounds● Sound is a pressure

wave that propagates in a medium at a peculiar velocity related to the medium density

● ULTRASOUNDS: acoustic energy with a frequency above human hearing (2Hz-20KHz)

● Medicine uses ultrasounds in diagnostic imaging technique (diagnostic sonography or ultrasonography)

In ultrasonography the wave-properties of sound are the key ingredients of the imaging techniques

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Wave properties of sound● λ=c

sν, where c

s is the sound velocity in the medium

● Reflection (eco)

● Refraction

Acoustic impedance (Z) of a medium

● Scattering● Absorption

(heat)

Attenuation (dB/mm)I=I

0exp[-μx] (intensity att. coefficient)

μ=μa+μ

s with μ

s/μ 0.1-0.3 (liver)

→ conversion into heat dominates→ attenuation is frequency related:

Approx: “n” Mhz – “n” dB/mm

(Reflection fraction)

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The transducer● Electrodes● Piezoelectric crystal

– dimensions → resonance freq.● 1.8 mm: Ʋ=1Mhz● 0.18 mm: Ʋ=10 MHz

● Backing block (absorber)

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Basic Imaging System in modern ecography

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The ecographic image

Image Properties

● Angular resolution

● Axial resolution

● Frame Rate

● Maximum range r: rmax

= vsT/2

● The transducer emits (and receives) short pulses of sonic energy

● The imaging system then waits a time T before sending out another pulse

● During this time T any echoes from obstacles in the path are reflected and received by the transducer

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Angular Resolution IA transducer, or array of transducer, can have circular or rectangular aperture

The angular resolution is the angular beam width. Quantification:

● Study the angular energy distribution and beam pattern obtained by collecting with a receiver at a distance d and angle θ the sound emitted with a frequency f

c by a

transducer of aperture a

Angular distribution of acoustic energy measured by the receiver

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Angular Resolution II● The beam pattern in the

ideal case (and “far field” approx) is a sinc function

● The beam width (i.e. Angular resolution) can be defined as the angular distance between the first zeros of the function

● ΔΘ=2 arcsin(vs/af

c)

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Angular Resolution II

HIGH ANGULAR RESOLUTION REQUIRES

LARGE TRANSDUCER APERTURE AND

FREQUENCE

● The beam pattern in the ideal case (and “far field” approx) is a sinc function

● The beam width (i.e. Angular resolution) can be defined as the angular distance between the first zeros of the function

● ΔΘ=2 arcsin(vs/af

c)

Side lobes effects (image degradation) can be reduced (apodization):

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Angular Resolution of phased arraysPhased array ● Phased arrays are multiple

tranducers whose relative phase is electronically adjusted in order to scan automatically the area of interest

● The bam pattern is not simmetrical anymore, and the beam width is a function of the beam direction

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Axial Resolution I

● We consider a pulse of duration ᐃt and frequence fc

● The axial resolution ᐃr, given by ᐃr = vs x ᐃt, is related

to the bandwidth B of the transducer

● Which frequencies are required to support such a pulse?

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Axial Resolution II● We can study the Fourier Transform of the pulse

● Its FT is the sinc function, with most of the energy contained within the two main lobes centered in ±f

c

● Most of the energy of the pulse can be trasmitted if the transducer has a bandwidth extending to the first zeros of the sinc function (that occurs when ᐃfᐃt=1)

● B = 2ᐃf = 2/ᐃt = 2vs/ ᐃr

FT of the pulse

● High axial resolution ᐃr=B/ 2vs:

increase B → decrease ᐃt

● BUT: at least 1 cycle: ᐃt≥1/fc → B⩽2 f

c

● increase B → decrease ᐃt → incr. fc

● Usually in ultrasonography: B=0.2-0.5 fc

HIGH AXIAL RESOLUTION REQUIRES HIGH FREQUENCIES

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Optimal frequency: a compromise● Both angular and axial resolution increases with increasing

frequencies

● But: better resolution means smaller penetration!

Circular transducer casef-number: ratio of the focal lenght of the transducer to its diameter

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Ultrasonography in practiceIs a very important tool for medical diagnostic used in

many different fields:

– Pregnancy

– Gynaecology

– Neurology (carotid)

– Andrology

– Cardiology (eco doppler)

– Gastroenterology

– Urology

– Cardiovascular system (intravascular ultrasound)

– Oftamology

– DermatologyHigher frequencies allowed!

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Ultrasonography in dermatology● High frequencies can be used in other

medical fields, such as dermatology (since 1979)

● Well established (diagnostic): 7.5, 15, 20 MHz

● Under study 100-150 MHz frequencies: fine structure

20 MHz ultrasonography:

● Axial res. 80 μm

● Lateral res. 200 μm

● Max depth: 1 cm

Typical use in diagnostic:

● Attenuation: in homogeneous structures (cysts) is smaller

● Skin thickness measurements

● Morphological change of limph-nodes

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Ultrasonography in oftamology● 10 - 20 MHz probes for diagnostic

● Deep structure of the eye (posterior segment)

● Important parameter: depth of field (axial range around the focus where the echo amplitude is at least ½ max) → simultaneous imaging of anterior and posterior segment of the eye is not possible with good resolution!

– 10 - 20 MHz → dof 9mm – 5mm

– Suitable for posterior segment

● In oftamology, usually mechanically scanned single element - focused transducer are employed

● The research in this sector now is trying to apply arrays (linear or circular)

20 MHz, dof 5mm, axial res. 0.5 mm

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UBM: Ultrasounds BioMicroscopy● UBM is a technique that uses very high

frequency ultrasounds for the study of the eye at microscopic resolution

● Typical range is 50-100 MHz

● The axial resolution is of the order of hundredth of μm

● The penetration is ∼mm

● Typical diagnostic use:anomalies in the structure of outer parts of the eye (angle-closure glaucoma)

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UBM images● Used to study the outer layers of

the eye (with high resolution)

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What can we “listen” from this image?

- The cornea, lens and irid are well visible

- Some informations are displayed:

● Dimensions: 5x5mm

● T delay: 2.24 mm

● Gain: 73 dB

● Time Gain Compensation (5 dB/mm) → suggests a frequency above 50 MHz

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Some measurementsstep 1: “set scale”

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The vertical profile

0.5 mm0.5 mm

2 mm

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The axial resolution

The resolution is of the order of 0.04 mm

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The “background” is quite flat

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The lens aperture

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An example of LSF

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Last measure: the irido-corneal angle

22.3°

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A new technique... back to light● Comparison between UBM image and AS-OCT (Anterior

Segment Optical Coherence Tomography)

● AS OCT is based on principles of low coherence interferometry

UBM● 50 MHz ● lateral resolution 50 μm ● axial resolution of 25 μm● Tissue penetration 4.0 to 5.0 mm● The scanner produces 5.0 mm x 5.0 mm field ● scan rate 8 frames/s

AS OCT● 13 μm wavelenght● lateral resolution 60 μm ● axial resolution is 18 μm● The scan was 16.0 mm in diameter and 6.0 mm deep (tissue)● Image acquisition rate of 8 frames/s

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Conclusions● Ultrasonography is a very useful diagnostic tool

– In general is non invasive

– No collateral effects● However:

– Skilled operator needed

– The image quality (axial and agular resolution) requires high frequencies, limited by strong attenuation

– High resolution (tens of microns) images can be achieved only in superficial scans

● UBM is a common technique that uses very high frequency ultrasounds for the study of the anterior segment of the eye with ∼50μm resolution

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Bibliography● “Basic physics of Ultrasound Imaging”, J.E.Aldrich, 2007

● “Basic Principles of Ultrasound Imaging System Design”, notes by A.Seagar, 2002

● “Priciples of Medica Imaging”, K.K.Shung, 1992

● “Ultrasound biomicroscopy”, C.Pavlin, 1998

● “Advances in UBM”, F.S.Foster, 2000

● “Improved High-Resolution Ultrasonic Imaging of the Eye”, R.Silverman, 2008

● “A comparison of 10 MHz and 20 MHz ultrasound probes in imaging the eye and orbit”, S.A.Hewick, 2004

● “Ultrasonography in dermatology”, D.Dill-Muller, 2007

● “Comparison of AS-OCT and UBM for assessment of the anterior segment”, T.Dada, 2007

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Backup slides

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Acoustic Impedance of Body Tissues

Medium Z (kg m-2s-1)

Air 4.29 x 102

Blood 1.59 x 106

Water 1.50 x 106

Brain 1.58 x 106

Soft Tissue 1.63 x 106

Bone 7.78 x 106

Muscle 1.70 x 106

Skin 1.70 x 106

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Sound velocity

Medium v (m s-1)

Air 300

Blood 1570

Water 1500

Fat 1450

Soft Tissue 1540

Liver 1560

Bone 3300-4080

Iron 5000

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The Scans

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Scans