Download - Dr hab. Zbigniew Serafin, MD, PhD [email protected] · Medical Image Properties Spatial Resolution or High Contrast Resolution 2D projection medical images have three dimensions: •

Radiology an diagnostic imaging

Day 4

Dr hab. Zbigniew Serafin, MD, PhD

[email protected]

Medical Image Properties

mainly based on:

R. Dickinson: RSNA & AAPM Physics Curriculum: Module 7. 2


Spatial Resolution or High Contrast Resolution

2D projection medical images have three dimensions:

• length

• width

• gray scale

Spatial Resolution = ability to perceive two distinct objects in the spatial

dimensions of an image

→ are the objects still resolved as two distinct objects if they are smaller in

size or closer together spatially?

Ideal PSF Actual PSF 3



the resolution of an imaging system is measureable and limited

4 lp/cm

5 lp/cm

6 lp/cm

7 lp/cm

8 lp/cm

9 lp/cm

10 lp/cm

12 lp/cm

4



Blurring – due to specific mechanisms of an imaging system

motion – voluntary vs. involuntary (cardiac motion); best controlled by

acquisition time

CT imaging – slice thickness and partial volume averaging; structures that

are not perpendicular to slice plane will be blurred (amount of blur is

proportional to slice thickness and angle relative to slice plane)

5



Matrix Size and Field of View (FOV)

if FOV is constant: increases

the matrix size improves resolution

if matrix size is constant: decreasing

the FOV improves resolution

pixel size = FOV / # pixels

6 6

10242 pixels 642 pixels

322 pixels 162 pixels

C.F. Bushberg, et al. The Essential Physics of Medical

Imaging, 2nd ed., pp. 82.

6


Noise

noise is the random (stochastic)

component in the image

as signal increases, the noise also

increases but at a slower rate,

therefore, the relative noise decreases

with increasing signal

7


Noise

Signal-to-noise (SNR = „image quality”) is the inverse of the relative

noise; thus, as the signal increases, the SNR increases

• if the signal is doubled, the dose is also doubled, but the SNR (or

the QUALITY of the image) only increases by sqrt(2)

• to double the SNR, you must increase the dose to the patient by

a factor of 4

120 kVp 140 kVp

8


Contrast

= the difference in the image

grayscale between two closely

adjacent regions on the image

contrast in an image is a direct

result of image acquisition,

processing, and display

contrast between two structures

increases if there is:

• difference in thickness

• difference of linear attenuation

coefficients of tissues /

materials

9


Contrast

10


EXERCISE

How to get good quality CT images in obese patients?

Try to describe CT examination of the highest radiation dose.

What will be CT# of a pineal calcification using 0.6 mm and 5.0 mm

slice?

11

computers, PACS & RIS

mainly based on:

E. Samei, et al.: AAPM/RSNA Tutorial on Equipment Selection: PACS Equipment Overview. General

Guidelines for Purchasing and Acceptance Testing of PACS Equipment. RadioGraphics 2004.

Brent K. Steward: Computers in Medical Imaging – Chapter 4, Computer Networks, PACS and

Teleradiology – Chapter 17. 12


digital storage of images

images are usually stored as a 2D array (matrix) of addressable

data, I(x,y): I(1,1), I(2,1), … I(n,m-1), I(n,m); n = column, m = row

each addressable location of the image is called a pixel (picture

element) represented by one value (e.g., digital value, gray level or

Hounsfield unit)

total number of bytes/image = pixels/image ∙ bits/pixel‡ ∙ (1 byte/8

bits)

c.f.: Bushberg, et al., The Essential Physics of

Medical Imaging, 2nd ed., p. 71.

13


medical image processing

addition or subtraction, e.g., digital subtraction angiography (DSA)

spatial filtering

• smoothing (removing quantum mottle – noise)

• edge enhancement, e.g., computed radiography (CR)

reconstruction from projections

• back-projection, e.g., computed tomography (CT), single photon

and positron emission tomography (SPECT and PET)

• Fast Fourier Transform, e.g., magnetic resonance imaging (MRI)

calculation of physiological performance indices, e.g., nuclear

medicine

generation and manipulation of volumetric data sets, e.g., MIPs

image co-registration (“fusion”), e.g., CT and PET

14


Computer-Aided Diagnosis (CAD)

computer program that uses specific image processing algorithms

and decision threshold parameters to detect features in an image

likely to be of clinical significance in images

assist as a secondary reader to call attention to objects that might

have been overlooked

first implemented in mammography:

• masses

• microcalcification clusters

• architectural distortions

15


teleradiology

= transmission of images for viewing at sites remote from where

they are acquired and reporting back

not a medical procedure in EU

16


PACS – Picture Archiving and Communication System

inter- and intrainstitutional computational system that manages the

acquisition, transmission, storage, distribution, display, and

interpretation of medical images

image file standard – Digital Imaging and Communications in

Medicine (DICOM)

administrative system – Radiology Information System (RIS)

administrative system – Hospital Information System (HIS)

17


18


Storage

19


20


EXERCISE

What is the best way to export medical images to the referring

physician?

21

Characteristics of Sound

22

mainly based on:

Ultrasound. RSNA & AAPM Physics Curriculum: Module 15 by Renée Dickinson


23

The basics – how ultrasound images are formed?

Mechanical energy is transmitted into tissue producing vibrations

Energy propagates through the tissue

Time between pulse emission and echo return determines depth

Amplitude of the echo determines grey scale

“call and response” pulse-echo imaging

c.f. Dowsett, et al. The Physics of Diagnostic Imaging, 2nd ed., p. 512.


24

Propagation of Sound

Sound is mechanical energy

Particles in the medium transfer the mechanical energy (small back

and forth displacement); vibrational motion produces

Compression (high pressure = high amplitude signal)

Refraction (low pressure = low amplitude signal)


25

Wave parameters

Wavelength (λ) [mm or μm] – distance between compression and refraction

(distance b/w two repeated points on a sine wave)

Frequency (f) [cycles per second = Hertz (Hz)] – number of times the wave

oscillates through a cycle each second

o Infrasound – sound wave f < 15 Hz

o Audible – 15 Hz < f < 20 kHz

o Ultrasound – f > 20 kHz (generally, medical ultrasound is 2-10 MHz)

Period (1/f) [seconds] – time duration of one wave cycle


26 EZ

constf

fc

Ec

material c [m/s] Z

air 330 0,0004

water 1495 1,49

adipose tissue 1450 1,38

liver 1550 1,64

blood 1570 1,66

muscles 1620 1,72

bones 4080 3,75

c – speed of propagation

E – volume elasticity

ρ – density

λ – wavelength

Z – acoustic impedance

f – frequency

Important Concept:

a higher frequency beam has a shorter wavelength


27

Wave parameters

The wavelength and frequency determine resolution and attenuation.

o High frequency (small wavelength) → improved spatial resolution,

however, depth of penetration is reduced

o Low frequency (long wavelength) → increased depth of

penetration, but resolution is degraded

Constructive and destructive interference – mostly dependent on the

wave phase


28

Wave parameters

Pressure, intensity, dB Scale

Recall: mechanical energy

causes particle

displacements, which alter

local pressure in propagation

medium

Pressure amplitude – peak

max or min value from the

average pressure in the

absence of a sound wave

c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 471.


29

Wave parameters

Relative intensity and power are described in units terms decibel (dB)

In diagnostic U/S, the intensity of the incident pulse can be up to 1

million times more than the echo pulse

The log function “compresses” the large ratios and “expands” the

small ratios into a more manageable range

o A change in 10 dB → an order magnitude (10x) change in

intensity

o A change in 20 dB → two orders magnitude (100x) change in

intensity

Interactions of Ultrasound

with Matter

30

mainly based on:


Interactions of Ultrasound with Matter

31

Interactions are dependent on acoustic properties of matter

Acoustic Impedance

Interactions

o Reflection – occurs at tissue boundaries (acoustic impedance of

adjacent materials)

o Refraction – change in direction of transmitted energy

o Scattering – occurs by reflection or refraction; energy diffuses in

many direction (affects texture and grey scale of acoustic image)

o Absorption – acoustic energy is converted into heat (sounds

energy is lost)

o Attenuation – loss of intensity from absorption and scattering


32

Acoustic Impedance (Z)

Similar to the stiffness and flexibility of spring

Dependent on medium density and speed of sound

SI Units is 1 kg per m2 per second

material c [m/s] Z

air 330 0,0004

water 1495 1,49


liver 1550 1,64

blood 1570 1,66

muscles 1620 1,72

bones 4080 3,75


33


For the energy transfer between two adjacent mediums:

o A large difference in the impedance results in a large reflection of

energy

o A minor difference in the impedance allows continued propagation

of energy (little reflection at the interface)

o Example: soft tissue to air-filled lung – large ΔZ, beam almost

entirely reflected

o …whereas if Z1 ~ Z2, then only minor reflections occur

Important concept:

acoustic impedance gives rise to difference in transmission and reflection of U/S energy (basis for pulse echo imaging)


34


small Z difference

large Z difference


35

Reflection

Result of acoustic impedance

Reflection coefficient –

describes fraction of sound

intensity incident on a interface

that is reflected

Note: reflection depends on

angle of incidence



36

Reflection

for perpendicular

incidence: Intensity

reflection coefficient

RT

ZZ

ZZ

I

IR r

1

2

21

21

0

I – wave intensity

Z – acoustic impedance

R – reflected energy

T – transmitted energy

material c [m/s] Z

air 330 0,0004

water 1495 1,49


liver 1550 1,64

blood 1570 1,66

muscles 1620 1,72

bones 4080 3,75


37

Refraction

As with light, sound is refracted if the incident sound is not

perpendicular to an medium interface

U/S frequency does not change at the boundary

Speed of the sound (both transmitted and reflected) changes

Angles (measured relative to normal incidence at the boundary) of

transmission and reflection are determined by the change in the

speed of sound



38

Refraction

No refraction if…

o Angle of incident is normal

o Speed of sound is the same in

both mediums

Straight line propagation is assumed

in ultrasound

o Refraction ‘artifacts’ cause

shadows and enhancements

because the sound waves are

bent from their expected paths


39

Scattering

Smooth boundary between media; uniform medium

Irregular surfaces or media

Reflects fewer echoes directly back to the transducer

Can cause diminished strength (amplitude) in echoes because of

destructive interference



40

Scattering

Two cases for scattering:

o At the boundary – smaller

wavelengths cause the

boundary to become

‘rough’ and the reflections

become diffuse

o Or small object reflectors

in tissue – diffuse

scattering patterns are

characteristic of specific

organ or tissue structure


41

Scattering

Scatter from diffuse reflectors affects the signal amplitude

Scatter depends on:

o Number of scatters (small objects) per unit volume

o Acoustic impedance difference

o Size of the scatters

o Ultrasonic frequency (because frequency is relate to wavelength)

Hyperechoic – high scatter amplitude relative to avg background signal

Hypoechoic – lower scatter amplitude relative to avg background signal


42

Scattering

Hyperechoic

Hypoechoic

Isoechoic


43

Attenuation

Attenuation is the loss of acoustic energy (signal amplitude), results

from scatter and absorption (heat)

Absorbed acoustic energy

o Attenuation coefficient, μ [dB per cm], is relative intensity lost per

centimeter of travel

o U/S attenuation is approximately proportional to frequency

o Since the dB scale is logarithmic, the signal intensity is attenuated

exponentially with distance


44

Attenuation

Image Acquisition

45

mainly based on:


Image Acquisition

46

Ultrasound System

Beam former – generation of electronic

delays for individual transducer elements in

an array to achieve transmit/receive

focusing and beam steering

Pulser (transmitter) – electrical voltage for

exciting piezoelectric (PZT) elements;

controls output transmit power by an

applied voltage

Transmit/receive switch – synchronized

electronically with pulser

Scan converter / image memory

Display

Image Acquisition

47

Modes of Operation

A-mode: amplitude

o Single pulse echo

o Clinical Application – ophthalmology

B-mode: brightness

o Brightness is proportional to the signal amplitude

o Used in M-mode and 2D grey-scale imaging

o Generates a 2D image; covers a plane of interest rather than on single

line of transmit/receive

M-mode: motion

o Fixed transducer position and beam direction – measure of motion

patterns for anatomy along a single line

o Recent developments in Doppler U/S replaced the need for M-mode

Image Acquisition

48

Modes of Operation

A-mode: amplitude

o Single pulse echo

o Clinical Application – ophthalmology

B-mode: brightness

o Brightness is proportional to the signal amplitude

o Used in M-mode and 2D grey-scale imaging

o Generates a 2D image; covers a plane of interest rather than on single

line of transmit/receive

M-mode: motion

o Fixed transducer position and beam direction – measure of motion

patterns for anatomy along a single line

o Recent developments in Doppler U/S replaced the need for M-mode

Image Acquisition

49

A-mode

B-mode

M-mode

Image Acquisition

50

Transducers or Probes

A transducer produces and detects the U/S waves

Ceramic elements with electro-mechanical

properties

o Transmit – converts electrical energy (applied

voltage) to mechanical energy

o Receive – converts mechanical energy to

electrical energy

Major parts of a transducer

o Piezoelectric (PZT) elements – functional

component of the transducer

o Matching layer – reduces acoustic impedance

o Backing (damping) block – absorbs backwards

directed waves


Image Acquisition

51


Piezoelectric elements – functional component of the transducer

The piezoelectric effect

o Transmit mode – electrical energy is converted to mechanical (sound)

energy by physical deformation of the crystal structure

• Applying an alternating current to the crystal of a transducer causes it

to expand and contract (vibrate), producing sound at that vibrational

frequency

o Receive mode – conversely, mechanical pressure applied to the surface

of the crystal during receive mode is converted into electrical energy

• On reception of a sound echo, the crystal vibrates at the sound

frequency of the echo and produces an electrical current

Image Acquisition

52


Image Acquisition

53

Image Quality

Spatial Resolution – 3 distinct measures

o Axial – depth resolution

• Minimal distance between objects is ½ the spatial pulse length

• Depends on frequency & damping factor

o Lateral – resolution perpendicular to the beam path

• Depends on beam diameter (since beam diameter varies with depth,

the lateral resolution varies withdepth) and mechanical/electronic

focusing

o Elevational (slice thickness)

• Typically worst resolution for array transducers

• Volume averaging of acoustic details

Additional Techniques

54

mainly based on:

Ultrasound by Kalpana Kanal


55

Harmonic Imaging

Degrades axial resolution

Improves lateral spatial resolution

Best used in abdominal imaging, use lower frequency to begin with, then

switch to higher frequency harmonics for better image quality and less clutter

adjacent to the transducer


56

Contrast Imaging

High frequencies also arise due to:

o The vibration of encapsulated gas bubbles used as contrast agents

o The responses of microbubble contrast agents under low and high

pressure reveal nonlinear compression and expansion of the bubble

radius

SF6

SF6

SF6

SF6


57

Contrast Imaging


58

Doppler Ultrasound

Based on shift in frequency in an US wave caused by a moving reflector

(blood cells)

Objects moving toward the transducer - higher frequency and shorter

wavelength

Objects moving away from the transducer - lower frequency and longer

wavelength

If object moving perpendicular to the transducer, no change in the observed

frequency or wavelength


59

cos2 001 c

vffff

f – wave frequency

v – blood velocity

c – wave velocity

α – angle wave / blood flow


Doppler Ultrasound


60

Doppler Ultrasound


61

Color Doppler Ultrasound

CD imaging provides a 2D visual display of

moving blood in the vessels, superimposed

on the conventional gray-scale image

Typically, flow toward the transducer is

assigned red and flow away from the

transducer blue

The color intensity varies with flow intensity

Color Doppler can detect flow in vessels too

small to be seen by imaging alone

Spatial resolution of the color image is

much lower than that of gray-scale image


62

Power Doppler Ultrasound

PD permits detection and interpretation of

slow blood flow but sacrifices directional and

quantitative flow information

It uses the return Doppler signal strength

alone

It is more sensitive than standard color flow

imaging

The image signal does not vary with the

direction of flow

Enchanced sensitivity in the PD acquisition –

in areas perpendicular to the beam direction,

where the signal is lost in CD


63

Spectral Doppler Ultrasound

Plot of Doppler shift frequency spectrum versus time

Amplitude is encoded as gray-scale variation

The spectral waveform - audible signal and provides information about

o the direction of the flow

o how fast the flow is traveling (velocity)

o the quality of the flow (normal vs. abnormal)


64

Spectral Doppler Ultrasound

Pulsatility Index (PI) increases as flow is impeded by e.g. a stenosis proximal

to the site of measurement

Resistivity Index (RI) reflects the resistance to blood flow caused by

microvascular bed distal to the site of measurement. RI is altered by both

vascular resistance and vascular compliance

systolic

diastolicsystolic

mean

diastolicsystolic

mean

v

vv

v

vvRI

v

vv

v

vvPI

max

minmax

minmax


65

Doppler Ultrasound

Duplex / Triplex Scanning


66

3D / 4D Imaging

Duplex / Triplex Scanning

Applications

67

mainly based on:

Ultrasound by Kalpana Kanal

Applications

68

Contraindications

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Applications

69

Safety

The intensity at a specific point during a single pulse is the spatial peak pulse

average intensity (ISPPA), W/cm2

The intensity at a specific point averaged over a long period (many pulses) is

the spatial peak temporal average intensity (ISPTA), W/cm2

No bioeffects have been shown below an ISPTA of 100 mW/cm2

Applications

70

Safety

At high power levels, ultrasound can cause:

Likelihood of Cavitation (Mechanical Index, MI)- the creation and collapse of

microscopic bubbles

Small-scale fluid motions called microstreaming

Tissue heating occurs as a result of energy absorption and is the basis of

using ultrasound for hyperthermia treatment

Thermal Index, TI is the ratio of the acoustic power produced by the

transducer to the power required to raise tissue in the beam area by 1 deg C

mode Pressure amplitude [MPa] ISPTA [W/cm2]

B-scan 1.68 19

M-mode 1.68 73

PD 2.48 1 140

CD 2.59 234

Applications

71

Safety

At high power levels, ultrasound can cause:

Mechanical damage – cavitation the creation and collapse of microscopic

bubbles. Effects: capillary hemorrhage in lung and intestinal hemorrhage in

neonates and preterm babies

Tissue heating occurs as a result of energy absorption and is the basis of

using ultrasound for hyperthermia treatment

Thermal Index, TI is the ratio of the acoustic power produced by the

transducer to the power required to raise tissue in the beam area by 1 deg C

o Scan mode

o Exposure duration

o Tissue sensitivity

Non human experiments found that increases of ≥ 40 C for > 5 minutes can

cause developmental abnormalities in fetal tissues

ALARA

Applications

72

Common indications

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Applications

73

TCD

CNS

trans-fontanel us

spine

Applications

74

carotids

neck

lymph nodes

thyroid

Applications

75

thymus

chest

pleural cavity

ribs

Applications

76

kidney

abdomen

spleen

pancreas

Applications

77

pregnancy

pelvis

testicles

TVU

Applications

78

knee

MSK

muscles

Applications

79

What structures cannot be diagnosed with US?

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Download - Dr hab. Zbigniew Serafin, MD, PhD [email protected] · Medical Image Properties Spatial Resolution or High Contrast Resolution 2D projection medical images have three dimensions: •

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