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

Post on 20-Jun-2020

0 views

Category:

## Documents

Embed Size (px)

TRANSCRIPT

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

 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

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

• 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 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

 = 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 – 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

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