dr hab. zbigniew serafin, md, phd serafin@cm.umk medical image properties spatial resolution or high

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  • Radiology an diagnostic imaging

    Day 4

    Dr hab. Zbigniew Serafin, MD, PhD

    serafin@cm.umk.pl

  • Medical Image Properties

    mainly based on:

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

  • Medical Image Properties

    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

  • Medical Image Properties

    Spatial Resolution or High Contrast Resolution

     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

  • Medical Image Properties

    Spatial Resolution or High Contrast Resolution

    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

  • Medical Image Properties

    Spatial Resolution or High Contrast Resolution

    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

  • Medical Image Properties

    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

  • Medical Image Properties

    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

  • Medical Image Properties

    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

  • Medical Image Properties

    Contrast

    10

  • Medical Image Properties

    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

  • computers, PACS & RIS

    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

  • computers, PACS & RIS

    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

  • computers, PACS & RIS

    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

  • computers, PACS & RIS

    teleradiology

     = transmission of images for viewing at sites remote from where

    they are acquired and reporting back

     not a medical procedure in EU

    16

  • computers, PACS & RIS

    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

  • computers, PACS & RIS

    18

  • computers, PACS & RIS

    Storage

    19

  • computers, PACS & RIS

    20

  • computers, PACS & RIS

    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

  • Characteristics of Sound

    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.

  • Characteristics of Sound

    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)

  • Characteristics of Sound

    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

  • Characteristics of Sound

    26 EZ

    constf

    fc

    E c

    

    

     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 – wa

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