SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Practical MRI Physics

Disclosures  None

Drew A. Torigian, MD, MA Associate A i t Professor P f off Radiology R di l [email protected]

Outline  Fundamental concepts in MRI physics  Basic principles of MR image creation  Basics of MRI pulse sequences and image contrast  Other MRI tools and techniques

Nuclear spin  Atomic nuclei with an odd # of protons or neutrons have a quantum mechanical property called spin

Fundamental concepts in MRI physics

 Spin gives a nucleus – A magnetic moment  The spinning nucleus behaves like a magnet

– An angular momentum  The spinning nucleus behaves like a gyroscope when placed in a magnetic field

Gerlach W, Zeitschrift fur Physik 1922;7:353-355 Pooley RA, Radiographics 2005;25:1087-1099

Drew A. Torigian, MD, MA

1

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Nuclear spin

Net tissue magnetization

 Nuclei with spin can be imaged by MRI – Hydrogen 1H protons are most commonly utilized Nucleus

Spin

Gyromagnetic ratio (MHz/T) (/2)

Natural abundance

Concentration in human tissue

Hydrogen 1H

1/2

42.58

~100%

88 M

Deuterium 2H

1

6.53

0.015%

13 mM

Sodium 23Na

3/2

11.27

~100%

80 mM

Phosphorous 31P

1/2

1.131

~100%

75 mM

Oxygen 17O

5/2

-5.77

0.04%

16 mM

Fluorine 19F

1/2

2.627

~100%

4 mM

 An external magnetic field (Bo) causes protons to align both in parallel and anti-parallel – There are ~4 in 106 net excess protons in the lower energy parallel state at 1.5 T at room temperature  Based on Boltzmann’s Boltzmann s equation

– Yet, protons are highly abundant (~1022 per ml) – This leads to formation of an initial net tissue magnetization (Mo) oriented with Bo (albeit quite small)  This can be represented as a vector

Plewes DB, JMRI 2012;35:1038-1054

Zeeman P, Phil Mag 1897;43:226-239

Magnetic field (Bo)  Bo of 1.5-3.0 T is most often used in clinical MRI – ~30,000-60,000 times the Earth’s magnetic field – Most often created via a superconducting magnet

 Higher field strengths are desirable – Signal-to-noise ratio (SNR) is linearly proportional to Bo Mo

 However, there is a limit – Specific absorption rate (SAR), the rate of RF energy deposition in tissue (W/kg), is proportional to (Bo)2

Pooley RA, Radiographics 2005;25:1087-1099

Precession  A nucleus with spin placed within a magnetic field wobbles or precesses about the field’s axis – Due to the torque exerted upon the nucleus by the force of the magnetic field

 The Larmor equation

relates

– Angular frequency of precession (o) of a nucleus to the magnetic field strength (Bo)  The greater the magnetic field, the greater the precessional frequency

M

– Gyromagnetic ratio () is a constant for the nucleus

 Net tissue magnetization (M) also precesses about Bo at o Larmor J, Proc R Soc 1896-1897;60:514-515

Drew A. Torigian, MD, MA

Plewes DB, JMRI 2012;35:1038-1054

2

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Resonance

Excitation

 Nuclei undergo absorption/emission of energy when the frequency of the energy source/sink matches that of the nuclei

 Nuclear excitation occurs when an alternating magnetic field (B1) (with frequency o) is applied perpendicular to Bo via an RF pulse

– Analogous to keeping a swing continuously moving by pushing it with the right timing

– Moves more nuclei into higher energy anti-parallel state – Brings precessing nuclei into phase with each other

 Nuclear magnetic resonance (NMR) is an energy exchange

 Net tissue magnetization (M) will “flip” by an angle  to a more transverse orientation relative to Bo (as it precesses about Bo)

– Occurs with nuclear excitation and relaxation – Seen as a change in net tissue magnetization (M) – Changes in M over time can be described via classical mechanics by the Bloch equations

Rabi I, Phys Rev 1938;53:318-327 Gorter CJ, Physica 1942;9;591-595 Bloch F, Phys Rev 1946;70:460-474 Purcell E, Phys Rev 1946;69:37-38 Torrey HC, Phys Rev 1956;104:563-565

Signal detection

 = 0o Mo

Mz

 < 90o M Mxy

B1 off

B1 on

o

 = 90o

M

o

B1 on

 The transverse component of M (Mxy) also precesses about Bo (at frequency o) perpendicular to a receiver coil  This induces an alternating current in the coil – Via Faraday’s law of induction – Leads to a rapidly decaying MR signal called free induction decay (FID)

 The longitudinal component of M (Mz) does not contribute to the MR signal

Pooley RA, Radiographics 2005;25:1087-1099 Plewes DB, JMRI 2012;35:1038-1054

T1 relaxation  When the RF pulse (B1) is turned off – Nuclei return to the lower energy parallel state  Via T1 (spin-lattice or longitudinal) relaxation  Due to energy exchange of nuclei with surrounding environment (aka, the lattice)  Occurs at an exponential rate

Mxy o

– Net tissue magnetization (M) realigns with Bo  Mz recovers 63% of its initial value in the time of T1

Pooley RA, Radiographics 2005;25:1087-1099 Currie S, Postgrad Med 2012;0:1-15

Drew A. Torigian, MD, MA

Bloch F, Phys Rev 1946;70:460-474 Bloembergen N, Phys Rev 1948;73:679-712

3

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

T2* (and T2) relaxation

M

 When the RF pulse (B1) is turned off – Nuclei dephase from each other in the transverse plane via T2* relaxation (decay)

M

 T2* decay is due to – Time-varying spin-spin magnetic field interactions (T2 decay) – Static Bo inhomogeneities, magnetic susceptibility, and chemical shift effects (T2’ decay)

 T2* (and T2) decays at an exponential rate

– Mxy decays to 37% of its initial value in the time of T2*

Pooley RA, Radiographics 2005;25:1087-1099 Gibby WA, Neurosurg Clin N Am 2005;16:1-64

Bloch F, Phys Rev 1946;70:460-474 Bloembergen N, Phys Rev 1948;73:679-712

Relaxation Mxy = 0

Mxy = Mo

Mxy < Mo

Mxy = 0

 These relaxation processes occur simultaneously but independently  T2* < T2 0 – magnetic field is augmented  Ions, simple salts and chelates of some metals (e.g., Gd+3)

– Ferromagnetic (and superparamagnetic) –  >> 0 – magnetic field is greatly augmented  Fe particles Schenck JF, Prog Biophys Mol Biol 2005;87:185-204

Drew A. Torigian, MD, MA

Schenck JF, Prog Biophys Mol Biol 2005;87:185-204

14

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

T2W images  FS T2W RARE images – Chemically selective saturation pulse used for FS – Performed with respiratory triggering – Useful for detection and characterization of lesions  Malignant lesions tend to have SI similar to spleen

– Useful to see internal architecture of lesions and organs

T2W images  High SI – Simple fluid – Fat (with FSE technique)

 Low SI – – – – –

Gas Cortical bone Fe Metal Fibrous tissue SIs listed here are relative to skeletal muscle

Drew A. Torigian, MD, MA

15

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

FS T2W FSE image Peribiliary and renal cysts in setting of ADPCKD

FS T2W FSE image Metastatic carcinoid tumor

Heavily T2W images  Heavily T2W SSFSE images – Performed in a breath-hold – Fluid, hemangiomas, cysts, and other fluid filled structures will have very high SI (> spleen) – Good for overall anatomy – Good for evaluating bowel – MRCP is very heavily T2W (with a very long TE)

Drew A. Torigian, MD, MA

16

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

Heavily T2W SSFSE image

1/24/2013

Delayed phase FS post-contrast T1W GRE image

Hepatic hemangioma

Very heavily T2W SSFSE MIP image (MRCP) Ampullary carcinoma

Drew A. Torigian, MD, MA

17

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

DW images

DW images

 Diffusion

 b value

– Based on thermally driven random (Brownian) motion of water molecules – Modified by interactions of water molecules with cell membranes and macromolecules – Mobile water protons acquire a phase shift resulting in loss of SI on DWI

– Determines sensitivity of the imaging sequence to water diffusion (i.e., the degree of diffusion weighting)

 Apparent diffusion coefficient (ADC) – Average A square off width idth off 3D Gaussian G i distribution di t ib ti off diffusion per unit time (mm2/sec) – Automatically calculated from each voxel of DWI using a mono--exponential fit mono  ADC = [ln (SIo/SI)]/b where SIo is for b = 0 and SI is for higher b values (Stejskal (Stejskal--Tanner equation)

Stejskal EO J Phys Chem 1965;42:288-292 Taouli B et al, Radiology 2010;254:47-66 Colagrande S et al, J Comput Assist Tomogr 2008:32:463-474

DW images

DWI/ADC map patterns

 Utilizes a modified T2W EPI sequence with diffusionsensitizing gradients  Performed with breath-holding, free-breathing, or respiratory triggering  2 – 3 different b values are selected – e.g., 50, 500, 800 for abdominal studies – 0 and 1000 for prostate gland studies

 ADC parametric map images are generated from DWI  Useful for detection and characterization of lesions

Stejskal EO J Phys Chem 1965;42:288-292 Taouli B et al, Radiology 2010;254:47-66 Colagrande S et al, J Comput Assist Tomogr 2008:32:463-474

Low b value DW EPI

High b value DW EPI

Stejskal EO J Phys Chem 1965;42:288-292 Taouli B et al, Radiology 2010;254:47-66 Colagrande S et al, J Comput Assist Tomogr 2008:32:463-474

Low b

High b

ADC

Benign lesion or lesional cystic/necrotic change Malignant soft tissue lesion (e.g., metastasis)

T2 shine through (e.g., cyst, hemangioma)

Taouli B et al, Radiology 2010;254:47-66

ADC map image

Metastatic colon carcinoma

Drew A. Torigian, MD, MA

18

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Low, mid, and high b value DW EPIs and ADC map image Lymphadenopathy due to SLL/CLL

Pre-contrast FS T1W images  Pre-contrast FS T1W GRE images – – – –

2D or 3D acquisition Chemically selective saturation pulse used for FS Imaging plane depends on organ/pathology of interest Useful for detection of macroscopic fat within lesions  a/w loss of SI

– Serves as baseline of comparison for post-contrast images

T1W FSE image

FS T1W GRE image

Mature teratoma (dermoid) of ovary

Drew A. Torigian, MD, MA

19

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Post-contrast T1W images  Post-contrast FS T1W GRE images – Keep all pulse sequence settings the same as for precontrast images as SIs are all relative – Can acquire images during various phases of enhancement – Useful for detection and characterization of lesions – Useful for assessment of vasculature

Pre-contrast FS T1W GRE image

Early venous phase post-contrast FS T1W GRE image

Delayed phase post-contrast FS T1W GRE image

Hepatocellular carcinoma

Drew A. Torigian, MD, MA

20

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Early arterial phase post-contrast FS T1W GRE MIP image Atherosclerotic disease – Leriche syndrome

Exogenous contrast agents  Extracellular fluid (ECF) agents – Most widely used – Composed of Gd+3 chelated to an organic compound – Shorten T1 (and T2) of adjacent water protons  Lead to increased T1W SI at low concentrations – r = specific relaxivity of contrast agent – C = concentration of contrast agent

 Lead to decreased T1W and T2W SI at higher concentrations

– Have similar pharmacokinetics as iodinated contrast – Have predominantly renal excretion – Generally 0.1 mmol/kg administered at ~2 mL/sec Weinmann HJ, AJR 1984;142:619-624 Hendrick RE, J Magn Reson Imaging 1993;3:137-148 Donahue KM, Magn Reson Med 1994;32:66-76 Donahue KM, J Magn Reson Imaging 1997;7:102-110

Other MRI tools and techniques  Other exogenous contrast agents

Other MRI tools and techniques

Drew A. Torigian, MD, MA

– – – – –

        

Hepatobiliary agents Hyperpolarized agents Reticuloendothelial (RES) agents Blood pool agents Targeted agents

Other pulse sequences and contrast mechanisms Dynamic contrast enhanced (DCE) MRI Magnetic resonance spectroscopy (MRS) Functional MRI (fMRI) Diffusion tensor imaging (DTI) Magnetic resonance angiography (MRA) Magnetic resonance elastography (MRE) Magnetic resonance thermography (MRT) Hybrid modalities – PET/MRI

21

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Hyperpolarized contrast agents  Involves magnetization of compounds containing certain nuclei (3He, 13C, etc.) – Leads to a marked increase in signal intensity – Allows for quantification of in vivo biochemistry – Mostlyy used in research settings g

 Limited by – – – – –

Short duration of signal Technical complexity of implementation Lack of standardization Potential toxicity High cost HP 3He MRI of airways and lungs Courtesy of Dr. W Gefter

PAO2

V/Q

Quantitative regional assessment of PAO2 and V/Q of human lungs using HP 3He MRI

Real time metabolic imaging of myocardium using HP 13C MRI Courtesy of R Rizi, PhD

Golman K, Petersson JS. Acad Radiol 2006;13:932-942

Magnetic resonance spectroscopy (MRS)  Provides quantitative data regarding endogenous molecular composition – Most often for 1H, but also for 31P, 13C, 23Na, 19F, etc. – Most M t often ft applied li d to t assess brain b i lesions l i

 Peaks result from changes in o of nuclei due to electron shielding in different microenvironments  Limited by – – – –

Drew A. Torigian, MD, MA

Sensitivity (mM range) Technical complexity of implementation Lack of standardization Sampling error (voxel size and placement)

22

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

Pretreatment multiparametric MRI of cerebral GBM Courtesy of Drs. S Wang, G Moonis, ER Melhem

Functional MRI (fMRI)  Brain activation imaging  Mostly used for research purposes  Requires – A task for the patient to perform (stimulus) – Fast readout sequences

 Based on blood oxygenation level-dependent (BOLD) T2*W contrast – Related to tight coupling between neuronal activation and blood flow – Leads to increased oxyHgb/deoxyHgb – Leads to increased SI  Since deoxyHgb (which decreases SI by inducing spin dephasing and shortening T2*) decreases

fMRI – First-person-shooter-video-gamers had significantly higher brain activations in dorsolateral prefrontal cortex compared with controls when shown game images Montag C et al. Biol Psychol 2012;89:107-111

PET/MRI  “One-stop shop” structural-functional-molecular imaging assessment  Combines the strengths of each modality – MRI: high contrast resolution, functional imaging sequences, no ionizing radiation – PET: high sensitivity, wide variety of radiotracers – PET/MRI: improved quantification  Partial volume correction  Motion correction  Improved anatomical localization of uptake sites

Torigian DA, Radiology 2013 in press

Drew A. Torigian, MD, MA

23

SNMMI 2013 Midwinter meeting CMIIT/PET CoE Joint Symposium

1/24/2013

PET/MRI  Challenges exist

Breast cancer (oncological)

Brainstem nuclei (neurological)

– – – –

Difficulties a/w MRI attenuation correction High cost of instrumentation Workflow issues Need for technologist and physician training in PET and MRI – Need for standardized guidelines regarding image acquisition and image interpretation/analysis

 Future research is needed to evaluate incremental benefits and costs relative to existing techniques Cardiac sarcoidosis (cardiovascular)

Osteomyelitis (musculoskeletal) Torigian DA, Radiology 2013 in press

Torigian DA, Radiology 2013 in press

Summary  MRI provides versatile structural and functional imaging capabilities  A basic understanding of MRI physics, image creation, and the available tools it offers is essential to use MRI to its full potential  MRI is complementary with molecular imaging techniques such as PET imaging

Drew A. Torigian, MD, MA

24