Imaging for Physicists

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ESTRO Course Book Imaging for Physicists

13 – 17 September, 2015 Leiden, The Netherlands

NOTE TO THE PARTICIPANTS

The present slides are provided to you as a basis for taking notes during the course. In as many instances as practically possible, we have tried to indicate from which author these slides have been borrowed to illustrate this course. It should be realised that the present texts can only be considered as notes for a teaching course and should not in any way be copied or circulated. They are only for personal use. Please be very strict in this, as it is the only condition under which such services can be provided to the participants of the course.

Faculty

Uulke van der Heide

Disclaimer

The faculty of the teachers for this event has disclosed any potential conflict of interest that the teachers may have.

Programme

Day 1

Sunday 13 September

09.00 – 09.50 Welcome and introduction

Uulke van der Heide, NL

09.50 – 10.40 10.40 – 11.10 11.10 – 11.50 11.50 – 12.40 12.40 – 13.10 13.10 - 14.10 14.10 – 15.00 15.00 – 15.40 15.40 – 16.10 16.10 – 18.00

MRI physics: basic principles

Eirik Malinen, NO

Coffee break

MRI physics: contrast formation MRI physics: space encoding

Tufve Nyholm, SE Gary Liney, AU

Questions and Answers (plenary) (MR physics)

Lunch

MRI physics : equipment

Gary Liney, AU

PET physics : basic principles

Daniela Thorwarth, DE

Coffee break

CASE / ASSIGNMENT

Day 2

Monday 14 September

08.30 – 09.20 09.20 – 10.10 10.10 – 10.30 10.30 – 11.20 11.20 – 12.10 12.10 – 12.40 12.40 – 13.40 13.40 – 14.30 14.30 – 15.10 15.10 – 15.40 15.40 – 17.00 08.30 – 09.20 09.20 – 10.10 10.10 – 10.30 10.30 – 11.20 11.20 – 12.10 Day 3

Uulke van der Heide, NL Uulke van der Heide, NL

MRI geometrical artifacts I MRI geometrical artifacts II

Coffee break

Pet physics : image reconstruction, contouring

Daniela Thorwarth, DE Cynthia Ménard, CA

Applications : MRI in brain

Questions and Answers (plenary) (MR physics)

Lunch

Functional imaging MRI

Gary Liney, AU Piet Dirix, BE

Applications: MRI in gynaecology RT

Coffee break

CASE / ASSIGNMENT

Tuesday 15 September

Applications: MRI in prostate

Cynthia Ménard, CA

Applications: MRI-guided radiotherapy for head-neck cancer

Piet Dirix, BE

Coffee break

CT physics 3D and 4D imaging

Koos Geleijns, NL Koos Geleijns, NL

CT physics and advanced applications

Free afternoon

Day 4

Wednesday 16 September

08.30 – 09.20 09.20 – 10.10 10.10 – 10.30 10.30 – 11.20 11.20 – 12.10 12.10 – 12.40 12.40 – 13.40 13.40 – 14.30 14.30 – 15.10 15.10 – 15.40 15.40 – 16.30 16.30 – 16.40 16.40 – 17.30

PET tracers and applications

Daniela Thorwarth, DE Daniela Thorwarth, DE

Guidelines for PET imaging in Radiotherapy

Coffee break

Dynamic PET and CT imaging

Eirik Malinen, NO Tufve Nyholm, SE

MR Safety

Questions and Answers (plenary) (PET)

Lunch

MRI physics: fast scanning, volume sequences

Gary Liney, AU

Parallel Discussion with the faculty

Coffee break

PRESENTATIONS : CASE / ASSIGNMENT

Break

PRESENTATIONS : CASE / ASSIGNMENT (10 presentations, break 10 minutes)

Day 5

Thursday 17 September

09.00 – 09.50 09.50 – 10.40

In-room imaging with cone-beam CT and MRI Applications : CT and PET for radiotherapy of lung cancer

Tufve Nyholm, SE

Uulke van der Heide, NL

10.40 - 11.00 Coffee break 11.00 – 12.40 PRESENTATIONS : CASE / ASSIGNMENTS

12.00 – 13.00

Short evaluation of the course and closing

Faculty

Uulke van der Heide

UMC Utrecht Utrecht, The Netherlands u.vd.heide@nki.nl Iridium Cancer Network Antwerp, Belgium piet.dirix@gza.be

Tufve Nyholm

Umeå University Umeå, Sweden tufve.nyholm@gmail.com

Piet Dirix

Koos Geleijns

Leiden University Medical Centre Leiden, The Netherlands K.Geleijns@lumc.nl Ingham Institute for Applied Medical Research Liverpool, Australia Gary.Liney@sswahs.nsw.gov.au DNR – Norwegian Radium Hospital Oslo, Norway eirik.malinen@fys.uio.no Princess Margaret Hospital Toronto, Canada cynthia.menard@rmp.uhn.on.ca Uniklinik für Radioonkologie Tübigen, Germany daniela.thorwarth@med.uni- tuebingen.de

Gary Liney

Eirik Malinen

Cynthia Ménard

Daniela Thorwarth

basic principles Eirik Malinen

• All clinical applications of MRI today are based on magnetic properties of the hydrogen nucleus • Body tissues contains lots of water and fat, and hence hydrogen

• Stern-Gerlach experiment: → Atomic nuclei has a quantized magnetic Otto Stern Walter Gerlach

• Consider charge q A

in circular motion: • Rotating charged sphere with uniform charge: Magnetic moment: r v q Current: r2 qv t q i π = ∆ ∆= mvr L , L m2 q iA = = =µ

q =µ

S

m2

Spin!

• Nuclear spin is a form of angular momentum • Nuclear spin, I , is quantized in units of ℏ • Nuclear quantum number depends on nuclear configuration; I=1/2, 1… • Hydrogen has spin I=1/2, with spin projection numbers m I =+1/2 , -1/2; spin ‘up’ or ‘down’ • Magnetic moment is μ =γ I

γ

Nucleus Unpaired Protons Unpaired Neutrons Spin γ (MHz/T) 1 H 1 0 1/2 42.58 2 H 1 1 1 6.54 31 P 1 0 1/2 17.25 23 Na 1 2 3/2 11.27 14 N 1 1 1 3.08 13 C 0 1 1/2 10.71 19 F 1 0 1/2 40.08

• In an external magnetic field, the potential energy is: → Two energy states are possible • Zeema ffect B ⋅ −= µ pot E B 2 1 Bm I    γ = γ−= pot E

2 1

B  γ+

B

2 1

B  γ−

• Spin system under an external magnetic field exposed to electromagnetic radiation • Transitions from spin down to spin up or vice versa may occur if ℏ ω = ΔE pot = γ ℏ B ℏ ω ΔE pot = γ ℏ Β Isidor Isaac Rabi

= γ ℏ

B → ω=γ

B; resonance condition

• ℏ ω

• Resonance frequency, 1 H, B=1T: ω≈ → Without external field With external field With external field + electromagnetic radiation 43 MHz

• Spin transition probability is equal for up → down and down → up • How can a net energy absorption be observed? • Distribution of spins follows Boltzmann: • Difference increases with B kTB kT E e e N N pot / / γ  − ∆− ↑ ↓ = =

• Population difference generates a net magnetization • The more spins, the stronger the magnetization • Torque exerted on a magnet by a magnetic field: M 0 BMM τ × = = γ dt d

d

BMM τ × = = γ

dt

Felix Bloch

0 Md , BM td Md z x y y x = γ−= γ= ⇒ td Md , BM td

ω = ⇒

0 x

0 y

cos( M)t(M

t) sin( M)t(M , t) ω =

x

L

y

L

• ω L = γΒ ; Larmor frequency • Set of equations describing a precession B 0 z M)t(M =

Joseph Larmor

z

z

x

x

y

y All spins in phase with same Larmor frequency Spins out of phase

• How can the magnetization be altered? • Introduce oscillating (RF) magnetic field in the xy-plane z x B y =B 1 cos( ω t) B 0 z x ω = ω L

• The degree of which the magnetization is tipped relative to B 0 due to an excitation pulse • From Bloch’s considerations: • t: duration of pulse • B1: ~RF power 1 B 2 πγτ =θ z x θ M xy M z

• Fluctuating magnetic fields from the molecular environment may have Larmor frequency→ stimulated transitions may occur • After an RF-pulse, the z-component of M relaxes back to equilibrium via such stimulated transitions • Longitudinal relaxation, Spin lattice relaxation, T1 relaxation • Rate of relaxation: R1=1/T1

• The transverse component of the magnetization also decays • Local, microscopic field inhomogeneities causes each spin to precess with a frequency slightly different from ω L • An excitation pulse initially causes all spins to precess in phase, but a dephasing then occurs • transverse- or spin-spin relaxation; T2

• However, transverse relaxation is also caused by B 0 inhomogeneities and tissue magnetic susceptibility • Actual T2 time is denoted T2*: • T2*

z

z

90 ° pulse

x

x

y

y

y time Transversal

x z Longitudinal T1

T2*

x

• Bloch’s equations expanded with relaxation components; M xy /T2* and (M z -M 0 )/T1 • May be shown that: ) e1(M)t(M 1T/t 0 z − − =

T1=300ms

M63.0 M

1 T t

∞ = ⇒ = z,

z

T2*=100ms T2* T2

*2T/t eM)t(M −

=

xy

0,xy

xy,0 xy M37.0 M *2T t = ⇒ =

Brain CSF Fat

• Changes in magnetization give rise to a current in a wire loop (Faraday’s law of induction) • Receiver coil perpendicular to B 0 : x y z x y 90 ° pulse relaxation Coil signal coil coil

• Envelope of FID describes the T2*-decay: Fourier transform

Thank you for your attention!

MRI physics: Contrast formation Tufve Nyholm

Precession

Magnetic field

42.576 MHz/T

Flip

RF puls

Magnetic field

42.576 MHz/T

Relaxation ( Rotating coordiante system

T1 relaxation Parallel plane

T2 relaxation Transversial plane

B0

T1 relaxation

• Spin-lattice or longitudinal relaxation • Restoring longitudinal magnetization after RF excitation • T1 – Time until 63% of the initial magnetization M0 is restored

Adipose tissue – 240ms Spinal fluid – 4300ms Gray matter – 980ms White matter – 780ms Muscles – 880ms

980ms

T2 relaxation

• Spin-spin or transversial relaxation • Loss of transversial magnetization after RF excitation • T2 – time until 63% of the transversal magnetization is lost

Adipose tissue – 70ms Spinal fluid – 2200ms Gray matter – 100ms White matter – 90ms Muscles – 50ms

100ms

T2* relaxation

Higher field

Lower field

Spin-Echo sequence

T1 relaxation

T2 relaxation

Signal equation

Constant depending on • Coils • Temperature • etc

Proton density

T2 contrast

Minimize influence i.e. Long TR

Focus

TE

T2 contrast

Examples T2 Contrast

Adipose tissue – 70ms Spinal fluid – 2200ms Gray matter – 100ms White matter – 90ms Muscles – 50ms

TE=90ms

T1 contrast

TR

180

90

TE

M

Tissue with shorter T1 Tissue with longer T1

T1 Contrast

Focus

Minimize influence i.e. Short TE

TR

Intermediate T1

Long T1

Short T1

T1 contrast

Adipose tissue – 240ms Spinal fluid – 4300ms Gray matter – 980ms White matter – 780ms Muscles – 880ms

Examples T1 contrast

TR=450ms

Inversion-recovery (IR)

TR

180

180

180

90

TI

TE

M

Intermediate T1

Long T1

Short T1

IR

IR

Summary

T1 contrast

T2 contrast

TE - Short TR – Optimized

TE - Optimized TR – Long

Inversion recovery TI - Optimized

• Use for anatomical imaging • For pathology together with contrast agent

• Use for pathology • Use for anatomical imaging

Proton contrast

Minimize infludence i.e. Long TR

Minimize influence i.e. Short TE

Focus

Turbo spin echo Fast spin echo

180

180

180

90

Gradient echo (T2*)

TR

α

α

TE

α

α

Gradient

TR

Gradient echo

Small angle - reduces T1 weighting and yielding proton density weighting Large flip - yields T1 weighting Short TR - increases T2* weighting (residual transverse magnetization is dominant) Long TR - enhances T1 weighting Short TE - reduces T2* weighting and increases T1 or PD weighting Long TE - enhances T2* weighting

What kind of contrast?

A. T2w B. T1w C. T1w + contrast

Different contrasts

T2w

T1w

T1w+GD

Why is it often difficult to see gold markers in T2w spin echo sequences of the prostate?

A. T2w images do often have low signal  poor signal to noise B. The gold gives the same signal as the prostate at long echo-times C. T2w images do often have a too large voxel size D. The artifact is often small in spin-echo sequences

25%

25% 25% 25%

The gold gives the same ...

T2w images do often hav...

The artifact is often small ..

T2w images do often have..

T2w

T1w Gradient echo

Summary again

• T1 Weighting

• Maximizing T1  short TR • Minimizing T2  short TE • Maximizing T2  long TE • Minimizing T1  long TR • Minimizing T2  short TE • Minimizing T1  long TR

• T2 Weighting

• Proton weighting

Thank you

MRI Physics: Space Encoding

A/Prof Gary Liney 13 th September 2015 ESTRO Imaging for Physicists

Introduction

• MRI extremely flexible spatial localisation Orientation easily altered • Gradients used to modulate phase and frequency In-plane directions always ‘phase’ and ‘frequency’ • Signal is reconstructed with 2D or 3D Fourier Transformation

Spin Echo Sequence

180 °

90 °

RF

G z G y

Slice Selection

Phase Encoding

G x Signal

Frequency Encoding

TE

An axial image..

Fourier Transform (FT)

• Time signal can be decomposed into sum of sinusoids of different frequencies, phases and amplitudes • Fourier series may be represented by frequency spectrum • Time and frequency domain data can be thought of as FT pairs s(t) = a 0 + a 1 sin( ω 1 t + ϕ 1 ) + a 2 sin( ω 2 t + ϕ 2 ) + …

Fourier Transform (FT)

 S1 has amplitude a and frequency f  S2 has a /2 and 3 f  S3 = S1 + S2  S3 is two sine waves of different frequency and amplitude The FT is shown

0.5 1.5 2.5 3.5

S1 S1 S2 S1 2 S3 3

-3.5 -2.5 -1.5 -0.5

A

FT Pairs

Delta ‘Top Hat’

FT

Sinusoid Sinc

FT

Time

Frequency

FT Pairs

Gaussian Lorentzian

FT

Gaussian Exponential

FT

Time

Frequency

Gradients

B γ

0 ω =

• Recall that the resonant frequency is proportional to field strength

0

dy dB G 0 dz dB G 0 dx dB G 0

=

x

• Magnetic gradient changes B 0 strength over distance • In MRI a linear gradient changes the resonant frequency in a given direction field

=

y

(

) x xG B + =

=

ω

γ

z

0

Slice Selection

isocentre

B 0

y

z

x

B 0

ω 0

Slice Selection

Gradient in z-direction G z

isocentre

B 0

y

z

x

B 0

+ ∆ B

B 0

B 0

- ∆ B

+ ∆ ω

ω 0

- ∆ ω

ω 0

ω 0

Slice Selection

• Gradient used to change resonant frequency in slice direction • Excite spins using ( sinc-shaped ) 90 ° RF pulse containing a bandwidth of frequencies • Only a particular section of spins are excited into transverse plane • Signal has been discriminated in one dimension • Can change orientation, slice thickness and position

Slice Selection

frequency

G z

G z

RF pulse length/Bandwidth Centre frequency

z

Slice thickness

Slice positions

Slice Selection

Frequency Encoding

• Need to discriminate signal in-plane • Another gradient is used to produce changes in resonant frequency

Phase Encoding

• Need to still encode signal in remaining direction (y)  Use changes of phase • When a gradient is applied the spins will be at different phases once the gradient has been turned off • This is the role of the phase encoding gradient (next)

Phase Encoding

ω = γ B 0

G phase

G y

Y-direction →

time →

Phase Encoding

ω = γ (B 0

+yG y

)

z

G phase

G y

Y-direction →

time →

Phase Encoding

Y-direction → ω = γ B 0

G phase

G y

time →

Initially, all spins have same frequency

Apply phase encoding gradient

slower

unchanged

faster

After PE Gradient turned off All spins have same frequency again, but different phase

Faster

unchanged

slower

Apply Frequency Encoding Gradient

Phase Encoding

• Each pixel is assigned a unique phase and frequency • FT decodes unique frequency but only measures summation of phase • Individual phase contributions cannot be detected • Need multiple increments of PE gradient to provide enough information about phase changes • Number of PE increments depends on image matrix

Spin Echo Sequence

180 °

90 °

Resonance condition ω = γ (B 0 + zG z )

RF

G z G y

G z

G x

z

Spin Echo Sequence

z Increment gradient after RF pulse and before read-out

180 °

90 °

RF

G y

G z G y

G x

Spin Echo Sequence

180 °

90 °

z Apply gradient during read-out

RF

G x

G z G y

G x Signal

Scan Time

• Frequency encoding done at time of echo • Phase encoding done over many TRs • Scan time is given by

N PE

× TR × NEX

Multi-Slice Imaging

• Period between the echo and the next RF pulse is called dead time • Used to excite a separate slice • Multiple slices are acquired in each TR • Slice profiles are not rectangular leading to cross-excitation • Slices are acquired with gaps or interleaved

‘3D’ MRI

• True 3D volume rather than multiple 2D slices • A slab or multiple-slabs are selected • Phase encoding also in the ‘slice’ dimension Through-plane resolution can be comparable to in- plane Phase wrap in ‘slice’ direction • SNR is improved, scan time longer

N PE

× TR × NEX × N s

What is k-space?

• ‘k’ is wave-number: number of cycles per unit distance  Spatial analogue to ‘cycles per second’ (frequency) • k-space is the raw data  An array of numbers whose FT is the MR image • Each row in k-space corresponds to the echo data obtained from a single application of the PE gradient  Rows near centre correspond to low-order PE steps (small gradients)  Rows at edges correspond to high-order steps

What is k-space?

FT

k-space and image-space of the brain

What is k-space?

k PE

k FE

Phase encoding increments

1

2

3

2 3

1

Frequency encoding gradient

k-space

• All of k-space needs to be filled to create an image  Centre: bulk signal/contrast information  Edge: image detail • Individual cells do not correspond one-to-one with individual pixels in image • Each cell has information about every image pixel: explains why motion artefacts propagate through whole image

k-space

k y

FOV

k x

∆ k

FOV = 1/ ∆ k ∆ x = 1/FOV k

k-space

k y

FOV

k x

∆ k

FOV = 1/ ∆ k ∆ x = 1/FOV k

k-space

Full k-space

Centre k-space

Edge k-space

k-space trajectories

k y

k x

GRE or SE : one line of k- space per TR (usually 256, 512 lines) Image time = N phase × TR

EPI : all lines of k-space per TR (typically 64 or 128) Image time = TR

k-space trajectories

Radial k-space : Centre oversampled Motion compensation

Partial k-space

These techniques acquire part of k-space and ‘fill-in’ the rest due to conjugate symmetry

k y

Just over half data collected

k x

Partial Fourier : Also called fractional NEX Collects half of phase-encode steps and speeds up imaging

Partial Echo : Collects half of echo reducing the shortest possible TE

MRI Physics: Equipment

A/Prof Gary Liney 13 th September 2015 ESTRO Imaging for Physicists

Installation of New Scanner

RF Cage

• MRI inherently low (RF) signal technique • Faraday cage  All 6 sides enclosed in copper  Electromagnetic shielding  Examples microwave oven, coax cable • Integrity must be maintained Penetration panel Mesh window, waveguide Closed scan room door, no fluorescent lights

RF Cage Construction

Waveguides

Penetration Panel

Mesh Window

Door surround

The MRI Controlled Area

Quench pipe

O 2

alarm

5 Gauss Line

Control Room

Pressure release hatch

The Inner Controlled Area

Scan Room

30 Gauss

Cabinet (Equipment) Room

RF

Heat Exchanger

Gradients

MNS

MRI Equipment: Overview

Plus: Peripheral equipment RT Specific equipment Test Objects

Patient Bore

short & wide bore

music

internal lighting and ventilation

panic button & intercom

windings in cryostat

shielding

gradients

vacuum

detachable table

RF body coil

+shim coils

Example Specifications

Shielding

Passive and active 0.2 (40 cm DSV)

Homogeneity (ppm) Field stability (ppm/hr)

<0.1

Cooling

Liquid helium only

Magnet specifications for Siemens Avanto 1.5 T

Boil-off (l/hr) Helium refill Shim plates Active shim

0

10 years 16 x 15

3 linear terms (20 coils) 5 2 nd order (32 coils)

Mass (tonnes)

5.5 2.5 4.0

Radial (x,y) 5 Gauss Axial (z) 5 Gauss Minimum area (m 2 )

<30

Example Specifications

RF channels

8,18,32

Bandwidth (MHz)

1

RF & Gradient specifications for Siemens Avanto 1.5 T

Gradient amplitude (mT/m)

33, 40, 45

Slew rate (mT/m/ms)

125-200

Host computer Memory (GB)

2 x Pentium IV

2

Hard drive (GB)

73 GB (images)

Computer specifications for Siemens Avanto 1.5 T

Image processor speed 2.2 GHz Reconstruction (ips 256 2 matrix) 1002

Magnet • Application

Whole body (head only) & peripheral systems

• Type

Permanent, resistive, superconducting

1987: Elscint’s Gyrex System

• Orientation

Horizontal, vertical field

• Design

Tunnel-short & wide bore Open

Philips’ vertical HFO System

‘Open-ness’

Dedicated systems

Whole-body systems ..shorter & wider

Field Strength

‘NMR’ systems

Static Field (B 0 ) • Low sensitivity requires high field • 1 Tesla = 10,000 Gauss 0.3-0.7 G Earth’s field • Projectile effect • Mostly superconductors field decay: 5-10 G y -1 field stability: <0.1 ppm h -1 y

z

B 0

x

Superconductors

• Niobium-Titanium • Cryostat

Double dewar with nitrogen/helium Cryoshielding helium only

• Cryogens

Cryostat

Quench Pipe

Replenish due to Boil off zero boil off/cryogen free

• Quench

Expensive & safety risk! Vent pipe, oxygen monitor

Homogeneity

Uniform imaging volume at isocentre

Off-centre imaging? RT Planning?

40 cm

e.g. DSV 40cm

= 0.2 ppm

(at 1.5 T): 0.2 x 63.87 MHz = 12.8 Hz

http://en.wikipedia.org/wiki/File:Finite_Length_So lenoid_field_radius_1_length_1.jpg#/media/File: Finite_Length_Solenoid_field_radius_1_length_ 1.jpg

• Magnet is shimmed at installation- additional (dynamic) shimming may be required

Shim Demo

Demo

Real Time Demonstration of Shimming

Fringe (stray) Field

• Scanner ‘footprint’

> 30 G Stainless steel, non- ferromagnetic objects < 30 G ECG monitors, unrestrained ferromagnetic objects < 10 G Credit cards, x-ray tubes

Credit cards erased at 10 G Safety limit is ‘five gauss line’ • 7 Tesla scanner has 23 m 5 G line Passive & Active shielding • Radial & axial components Typically axial 1.6 times larger • May be measured with handheld gaussmeter

< 5 G

Pacemakers, general public

< 3 G

Moving cars etc

< 1 G

TVs, CT & PET scanners

< 0.5 G Railways, gamma cameras

5 G line with Active shielding

+ Passive shielding

Gradients (db/dt)

G z

= dB 0

/dz

isocentre

• 3 orthogonal or in combination • High amplitudes -Resolution -DTI • Fast switching rates -Faster scans

y

z

x

B 0 ω 0

B 0

- ∆ B - ∆ ω

B 0

+ ∆ B

+ ∆ ω

ω 0

ω 0

Gradients

• Gradient waveform trapezoidal • Amplitude, Rise time, Slew rate e.g. 10-50 mT/m, 200 µ s & 20-150 T/m/s • Linearity Distortion for RT planning? GradWarp or similar in 2D/3D ?

Max amplitude plateau

Slew Rate (T/m/s) = Amplitude (mT/m) Rise Time ( µ s)

Rise time

Gradients

Maxwell Pair

Separation = r √ 3

Golay Coils

Linear between central

arcs

Optimised inductance ‘fingerprint’ coils

Gradients

Manufacturer

Field Strength (T)

SPL (dB(A))

• Eddy currents degrade imaging pre-emphasis (compensation) Active shielding • Lorentz force causes vibrations

Philips

1.5 1.5 1.5 3.0 3.0

112 106 110 118 113

Siemens

GE

Varian Bruker

First Field

Noise & reduction methods

thuMb

Motion

seCond

Current

Peripheral Nerve Stimulation (PNS)

cardiac

• Faster switching = faster imaging • Stimulation real issue • Reilly estimates (right) • Solutions: Parallel imaging (‘Coil Encoding’) Twin gradients

nerve

RF Coils (B 1 )

• Coil Usage:

Transmit and/or receive at

resonance • Properties

36.5

36

Cable loss, loading

35.5

Core Skin

Q factor ( ω / ∆ω ) Efficiency 1-Q L

35

33.5 Temperature ( o C) 34 34.5

/Q 0

Filling factor

33

32.5

• RF heating effects (SAR)

32

0

5

10

15

20

25

Time (min)

RF Chain  DAC Turns digital signal to analogue for RF transmission  Double balanced mixer Produces amplitude modulated RF waveform  RF power amplifier  RF Coil(s) transmit/receive signal  Pre-Amp  Phase sensitive demodulator Removes RF waveform from detected signal  Low Pass Filter  ADC Digitisers signal to be processed by computer

RF Coils: Signal Characteristics

2 a x a I π

2

0 µ

(

) 2 2 4 2/3

B

=

Theoretical cylinder coils

+

π

SNR

a

Body Coil: Poor SNR Excellent uniformity

surface coils Surface Coil: Excellent SNR close to coil Poor uniformity

Distance

Finite Element Modelling used for complicated designs

RF Coil Designs

• Surface coils • Cylindrical coils

Sinusoidal currents around surface gives homogenous B 1 Saddle, birdcage (‘distributed capacitance’) with more conductors approximate this • Solenoid useful in vertical fields (Philips HFO)

B 1

saddle

surface

birdcage

solenoid

RF Coils

• Typical Scanner Configuration:  Integrated body coil  Head coils (linear for QA)  Torso Coil  Surface coil  Specialist coils e.g. wrist, breast

Coil Arrays

• Extend surface coil coverage Small coil excellent SNR • Overlap to prevent mutual inductance • Separate Rx channels Noise not correlated, further increase SNR • Can be used in parallel imaging*

* Covered in ‘fast scanning, volume sequences’

Quadrature Coils

imaginary

• Linear polarisation- only half RF power effective • Circularly polarised Orthogonal coils at 90 ° phase • Efficient transmission Power halved (RF heating) • Receiver coils SNR increases √ 2

real

RF Coils: Other

• Dual Tuned

Multi-nuclear spectroscopy proton MRI & other MRS Broadband amplifier e.g. GE’s OpTix system Digitised in scan room, optical transmission SNR increase by 27%

• Optical RF Chain

B 1

Uniformity

• Surface coil uniformity problematic (ER coil prostate) • Commercial correction methods (e.g. SCIC) • In-house method: PD-W image to divide out inhomogeneites

original

corrected

Dielectric Effect

• At 3T λ ≈ 26 cm, comparable to patient • Conductive/resonance effect ‘B 1 Doming’ • Dielectric pad, test objects Body imaging restored (right) • Dual transmit body coil

RT Specific Equipment

 Increase in sophistication from ‘making do’ to dedicated equipment

Couch: Flat table-top (?RF coil in table) Magnet: wide bore RF coils: Use of diagnostic and/or dedicated equipment External lasers in MRI room Associated devices- markers, MR applicators…

RT Planning Scans

MRI often compromised by available equipment

Dedicated System (MR Simulator)

Dedicated RF coils

Laser bridge, Wide bore Flat table top, Coils in bed

Flex coils (2 x 4), table coil (32) plus long cabled body coil (18) Improved SNR & coverage H&N

The Future • Higher field strength • More RF channels

• Increase in MR-Simulators • Hybrid MR-Linac systems

7 Tesla system

64 channel H&N coil

The Australian MR-Linac

Positron Emission Tomography Physics - Basic Principles

ESTRO Teaching Course on Advanced Imaging Technologies September 13 – 17, 2015 in Leiden, NL

Daniela Thorwarth Section for Biomedical Physics, University Hospital for Radiation Oncology, Tübingen

Emission Tomography (PET)

PET imaging adds molecular information to morphology and function imaged with CT and/or MRI.

[ 68 Ga]DOTATOC PET/MR T2w MRI and PET (BrainPET) show small satellite in dorsal area of frontal sinus (detected on PET).

Boss et al, JNM 2010; 51.

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Basic principle of PET

 Positron emitters (β+) used as biomarkers  Positron-electron annihilation ⇒ Two γ-quanta with 511 keV each are emitted under approx. 180 °

 Coincidence detection in a detector ring

Radioactive Isotope

511 keV γ-quant

γ 1

β +

Electron

γ 2

511 keV γ-quant

PET

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Today: Combined PET/CT

PET/CT: combination of structure and function

First clinical PET/CT prototype (mid 1990s) [3]

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

State-of-the-art PET/CT Designs

Gemini series, Philips Healthcare Systems

Biograph series, Siemens Healthcare Solutions

Discovery series, GE Healthcare

Aquiduo series, Toshiba Medical Systems

Sceptre series, Hitachi Medical Systems

Anyscan series, Mediso

!

All PET/CT tomographs combine diagnostic PET and CT components and a dedicated patient support system. ESTRO Teaching Course Advanced Imaging: PET – Basic Principles 5 ! Courtesy T. Beyer,

PET/CT

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Sinograms: Measurement of the activity distribution of a radioactive tracer. Raw data stored in sinograms

φ

y = φ

line of response (LOR)

x = offset

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Radial Sampling

Mapping from sampling projections to sinograms [2].

Transaxial field of view of a PET tomograph is defined by the acceptance angle in the plane.

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

2D-/3D-PET 2D-PET • Geometric collimation with septa

3D-PET • Projections at polar angles θ>0° measured • Increased sensitivity • Higher scatter fraction • Special reconstruction algorithms are necessary

• Data sampling only with θ=0° • Lower overall sensitivity • Lower fraction of scattered photons

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

2D/3D-PET acquisition

2D acquisition: the entire FOV is sampled.

3D acquisition: truncation of projections occurs for θ > 0°. This results in loss of data corresponding

the ends of the tomograph [2].

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Axial Sensitivity

Restriction of max. acceptance angle

2D axial sensitivity profile for a line source in air in a 16-ring tomograph [2].

Axial sensitivity variation for 3D acquisition in a 16-ring tomograph [2].

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Image Formation

(1) Emission scan (2) Normalization scans (one per plane in 2D) to correct for differential detector efficiencies and geometric effects related to the detector ring (3) Set of sinograms of attenuation correction factors to correct of photon attenuation (self- absorption or scattering) by the object

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Radiation detection

Desired for PET: 1. High stopping efficiency 2. Good energy resolution

Scintillation detectors

• Inorganic crystal that emits visible light photons after interaction of photons with detector. • # of scintillation photons is proportional to the energy deposited in the crystal. • Important properties for application in PET: • Stopping power for 511 keV photons • Signal decay time • Light output • Intrinsic energy resolution

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

applied in PET

NaI(Tl): sodium iodide deoped with thallium BGO: bismuth germanate (Bi 4 Ge 3 O 12 ) LSO: lutetium oxyorthosilicate doped with cerium(Lu 2 SiO 5 :Ce) YSO: yttrium oxyorthosilicate doped with cerium(Y 2 SiO 5 :Ce) GSO: gadolinium oxyorthosilicate doped with cerium(Gd 2 SiO 5 :Ce) ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

New: LYSO

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Photo-multiplier tubes (PMTs)

PMTs: photo-detectors used in scintillation detectors for PET

(1) Incoming photon deposits its energy at the photocathode, release of a photo-electron (2) Applied electric field accelerates the electron to the first dynode (3) Emission of multiple secondary electrons due to increased electron energy

(4) …

Good signal-to-noise ratio (SNR) Low quantum efficiency (QE) ~ 25%

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Detector Designs used in PET

One-to-one coupling: •

Single crystals glued to individual photo-detector Spatial resolution limited by discrete crystal size

Block detector design: •

Rectangular scintillator block sectioned by partial saw cuts of different depth into discrete elements Usually 4 attached PMTs

Standard block detector design scheme (from [2])

• •

Anger positioning

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Detector Designs used in PET

Anger detector: •

Large scintillator crystal glued to array of PMTs Weighted centroid positioning algorithm used to estimate interaction position within the detector

Block detector Siemens-CTI ECAT 951, 8x8 block BGO with 4 PMTs (from [2])

Block detector system + Anger logic [3]

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Detection

Coincidence time window: 2 τ

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Question: Timing Resolution

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ESTRO Teaching Course Advanced Imaging: PET I

Answer: Timing Resolution

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ESTRO Teaching Course Advanced Imaging: PET I

Time-of-flight (TOF) PET

• In addition to the coincidence information, the difference in flight time is registered

=∆ 2

x

t

c

• Better SNR

• Example: Δt=500ps Δx=? =∆

5.7 5.0

t c

∆⋅ ⋅

cm

=

(a) Localization without TOF, (b) with TOF. Townsend, PMB 2008 [3]

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Question: Time-of-flight

 Assuming the detector system of a PET scanner has a timing resolution of 500 ps. How long is the corresponding sector on a LOR, where the annihilation event was located?

1. 75 cm 2. 15 cm 3. 7.5 cm 4. 1.5 cm

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ESTRO Teaching Course Advanced Imaging: PET I

Question: Time-of-flight

 Assuming the detector system of a PET scanner has a timing resolution of 500 ps. How long is the corresponding sector on a LOR, where the annihilation event was located?

1. 75 cm 2. 15 cm 3. 7.5 cm 4. 1.5 cm

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ESTRO Teaching Course Advanced Imaging: PET I

Detected Events in PET  Detection event is valid (= prompt event) if ● Two photons are detected in coincidence window ● LOR is within valid acceptance angle ● Energy of both photons within selected energy window

B

A

C

τ ≈10 ns

2

True coincidence (A)

, scattered coincidence (B) , random coincidence (C)

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Prompt Events  Single event

● single photon is counted by detector (1-10%)  True coincidence ● event derives from single positron-electron annihilation ● both photons reach tomograph without interaction  Random coincidence ● two nuclei decay at approximately the same time ● random event count rate (R ab ) between two detectors a and b :

2 ∝ ⋅ N NN R b a

2 = τ

ab

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Prompt Events (II)  Scattered events

● One or both photons detected have undergone a Compton interaction

● Loss in energy and change in direction ● Due to poor energy resolution, many scattered photons cannot be discriminated ● Wrong LOR assigned  Multiple (triple) events ● Three events from two annihilations detected ● Event is disregarded ● Proportional to count rate

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Performance of PET Systems

 Sensitivity

counts

   At c S = ⋅

  

=

sec

kBq

● Good systems reach S=7-9 counts/(sec.kBq)  Spatial Resolution  Energy Resolution  Count Rate Performance  Scatter Fraction

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Spatial Resolution Determined by full width half maximum (FWHM) of point spread function (PSF): )2 ln( 8 σ = FWHM

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Energy Resolution  Statistical uncertainty of energy determination due to limited light yield of scintillator crystal  Two methods for determination of energy resolution: ● Single event energy resolution ● Coincidence (i.e. both events) energy resolution

FWHM = 16.4/19.6/21.6%

Energy spectra for single photons for a BGO PET system [2].

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Count Rate Performance  Ratio Trues/Randoms unbalanced for high Activities  Processing of detected photons takes finite time  Noise Equivalent Count Rate: T 2

T : trues S : scattered R : randoms f : rel. area of object on proj. surface

NECR

=

2 ++

fR ST

Count rate curves for true, random and multiple coincidences. Derived curves for expected and noise equivalent count rate [2]. Data recorded on a CTI ECAT 953B PET

Maximum image quality

Low image contrast

using a 11 C-filled cylinder in water.

Noisy image

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Scatter Fraction  Fraction of the total coincidences recorded in the photopeak window that have been scattered  Sources of scattering ● Scattering within the object containing the radionuclide

● Scattering off the gantry components (lead septa/side shields) ● Scattering within the detectors

ESTRO Teaching Course Advanced Imaging: PET – Basic Principles Log-lin count rate profiles of line source in air/water show additive scatter component outside of central peak [2].

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Scatter Fraction  Fraction of the total coincidences recorded in the photopeak window that have been scattered  Sources of scattering ● Scattering within the object containing the radionuclide

● Scattering off the gantry components (lead septa/side shields) ● Scattering within the detectors

 Scatter fraction can be reduced by ● TOF

● Usage of a powerful iterative reconstruction method ● Shielding of scattered photons by septa and endshiels ● Small coincidence window ● Good energy resolution

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Summary

• Positron emitter used as radioactive tracer in the patient • Use of collimators to generate ‚image slices‘ • Detection of γ-quanta in scintillation crystal • Better resolution due to Anger logic • Signal amplification using PMTs • Image reconstruction

Literature [1] DL Bailey, JS Karp, S Surti. Physics and Instrumentation in PET. In: Positron Emission Tomography: Basic Science and Clinical Practice. Editors: PE Valk, DL Bailey, DW Townsend, MN Maisey. Springer London 2003, pp. 13-39. [2] DL Bailey. Data Acquisition and Performance Characterization in PET. In: Positron Emission Tomography: Basic Science and Clinical Practice. Editors: PE Valk, DL Bailey, DW Townsend, MN Maisey. Springer London 2003, pp. 41-61. [3] DW Townsend. Multimodality imaging of structure and function. Phys Med Biol 2008; 53: R1-R39. [4] B Sattler, JA Lee, M Lonsdale, E Coche. PET/CT (and CT) instrumentation, image reconstruction and data transfer for radiotherapy planning. Radiother Oncol 2010; 96: 288-297. Review.

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ESTRO Teaching Course Advanced Imaging: PET – Basic Principles

Imaging for Physicists Artifacts 1 Uulke van der Heide

Artifacts in MRI

Artifacts in MRI

• An image artifact is any property or effect observed in an image that does not appear in the original object

• Images can be distorted in many ways – Signal loss – Deformations – Poor resolution – Ghosting – Aliasing – …..

• Consequences for use

– Interpretation is difficult – Geometrical accuracy may be compromised

Outline

Lecture 1 • Origin of geometrical artifacts – Fold-over artifacts – Ringing – Impact of field distortions

Lecture 2 • Measurements for characterizing geometrical accuracy – Phantom design – Characterizing gradient errors • Examples • Practical consequences • Summary

Origin of various artifacts

• Sampling of k-space

– Sample k-space in too large steps – Don’t sample high k-values

• Magnetic field errors

– Inhomogeneous B 0 – non-linear gradients – Susceptibility – Chemical shift

field

• Motion

Imaging artifact

• T 1

-weighted SE image of a brain

• What is wrong?

Sampling the MR signal

• Nyquist criterium: signal must be

sampled at at least twice the rate of the highest frequency component

Sampling the MR signal

• Nyquist criterium: signal must be

sampled at at least twice the rate of the highest frequency component

is not > 2f 2

F sampling

• If the signal contains higher frequency components, aliasing occurs

Resolve aliasing by increasing sampling frequency

• Nyquist criterium: signal must be

sampled at at least twice the rate of the highest frequency component

• By increasing the sample frequency, the higher frequency components can be resolved and aliasing is avoided

Field Of View covers entire object: no fold-over

k y

k x

FOV

Question: field of view

• What happens with the distance between lines in k-space, if you reduce the field of view by a factor of 2?

1. The distance between k-lines is increased by a factor of 2 2. The distance between k-lines is reduced by a factor of 2 3. The distance between k-lines remains the same (but the extent of k-space is reduced by a factor of 2

Field Of View covers entire object: no fold-over

k y

k x

1/FOV

FOV

FOV too small: fold-over

k y

k x

1/FOV

FOV

How to suppress fold-over artifacts?

k y

k x

1/FOV

FOV

• If NSA>1: Measure all k-lines (full FOV), but reconstruct only half of the image

How to suppress fold-over artifacts?

k y

k x

1/FOV

FOV

• If NSA>1: Measure all k-lines (full FOV), but reconstruct only half of the image

How to suppress fold-over artifacts?

k y

REST slab

k x

1/FOV

FOV

REST slab

• If NSA=1: Saturate the signal from outside the field-of-view with REST slabs

Saturate signal from outside FOV

k y

k x

1/FOV

FOV

Imaging artifact

• T 1 -weighted SE image of a brain • What is the difference between the left and right image?

Imaging artifact

• T 1 -weighted SE image of a brain • What is the difference between the left and right image?

Truncation errors (ringing)

• Imaging sharp edges requires high frequency components in k-space • Cutting off the high-frequency k-lines causes oscillations in the image

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