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The building blocks of Magnetic Resonance Imaging: hardware and...

The building blocks of Magnetic Resonance Imaging: hardware and excitation The building blocks of Magnetic Resonance David Norris Imaging: hardware and excitation Introductory electromag- netism MRI system components David Norris Excitation 1 / 31 What is a field? The building blocks of Magnetic Resonance Imaging: hardware and I A field is where a force operates. excitation David Norris Introductory electromag- netism MRI system components Excitation 2 / 31 What is a field? The building blocks of Magnetic Resonance Imaging: hardware and I A field is where a force operates. excitation David Norris I There are many kinds of forces and fields. Introductory electromag- netism MRI system components Excitation 2 / 31 What is a field? The building blocks of Magnetic Resonance Imaging: hardware and I A field is where a force operates. excitation David Norris I There are many kinds of forces and fields. Introductory electromag- I Any object with non-zero mass experiences netism gravitational attraction. You are now sitting in the earth’s MRI system components gravitational field. Excitation 2 / 31 What is a field? The building blocks of Magnetic Resonance Imaging: hardware and I A field is where a force operates. excitation David Norris I There are many kinds of forces and fields. Introductory electromag- I Any object with non-zero mass experiences netism gravitational attraction. You are now sitting in the earth’s MRI system components gravitational field. Excitation I Gravity is the weakest force, the next strongest are the electrical and magnetic forces. 2 / 31 Field of a bar magnet The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 3 / 31 The magnetic field of a current The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris I Electrical currents generate magnetic Introductory electromag- fields. netism MRI system components Excitation 4 / 31 The magnetic field of a current The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris I Electrical currents generate magnetic Introductory electromag- fields. netism MRI system I If the strength of the components current varies in time Excitation then so does the strength of the field. 4 / 31 The magnetic field of a current The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris I Electrical currents generate magnetic Introductory electromag- fields. netism MRI system I If the strength of the components current varies in time Excitation then so does the strength of the field. 4 / 31  The building blocks of Magnetic Resonance Imaging: We can shape the magnetic hardware and excitation field by shaping the wire. We David Norris use this effect to generate all Introductory the magnetic fields used in electromag- netism MRI. MRI system components Excitation 5 / 31 Faraday’s law The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 6 / 31 Faraday’s law The building blocks of Magnetic Resonance Imaging: A change in the magnetic hardware and excitation environment of a wire loop David Norris (i.e. the field lines from a Introductory magnet moving through the electromag- netism coil) will give rise to a voltage MRI system across its terminals and a components current will flow. Excitation Think of a bicycle dynamo! 6 / 31 Faraday’s law The building blocks of Magnetic Resonance Imaging: A change in the magnetic hardware and excitation environment of a wire loop David Norris (i.e. the field lines from a Introductory magnet moving through the electromag- netism coil) will give rise to a voltage MRI system across its terminals and a components current will flow. Excitation Think of a bicycle dynamo! 6 / 31 System components To perform MRI we need three very different forms of The building blocks of magnetic field: Magnetic Resonance Imaging: 1. The magnet which generates the main magnetic field, hardware and denoted by (B0 ). The orientation of the main magnetic excitation David Norris field is by convention along the z-axis. Introductory electromag- netism MRI system components Excitation 7 / 31 System components To perform MRI we need three very different forms of The building blocks of magnetic field: Magnetic Resonance Imaging: 1. The magnet which generates the main magnetic field, hardware and denoted by (B0 ). The orientation of the main magnetic excitation David Norris field is by convention along the z-axis. 2. The gradient set, which generates switched magnetic Introductory electromag- field gradients in (Bz ). There are three gradient fields netism giving a gradient in Bz along the x, y, and z-axes. These MRI system components are denoted Gx , Gy , Gz. Excitation 7 / 31 System components To perform MRI we need three very different forms of The building blocks of magnetic field: Magnetic Resonance Imaging: 1. The magnet which generates the main magnetic field, hardware and denoted by (B0 ). The orientation of the main magnetic excitation David Norris field is by convention along the z-axis. 2. The gradient set, which generates switched magnetic Introductory electromag- field gradients in (Bz ). There are three gradient fields netism giving a gradient in Bz along the x, y, and z-axes. These MRI system components are denoted Gx , Gy , Gz. Excitation 3. The radio-frequency system consisting of: 7 / 31 System components To perform MRI we need three very different forms of The building blocks of magnetic field: Magnetic Resonance Imaging: 1. The magnet which generates the main magnetic field, hardware and denoted by (B0 ). The orientation of the main magnetic excitation David Norris field is by convention along the z-axis. 2. The gradient set, which generates switched magnetic Introductory electromag- field gradients in (Bz ). There are three gradient fields netism giving a gradient in Bz along the x, y, and z-axes. These MRI system components are denoted Gx , Gy , Gz. Excitation 3. The radio-frequency system consisting of: 3.1 The transmitter and resonator which together generate a rotating field known as the (B1 ) field. 7 / 31 System components To perform MRI we need three very different forms of The building blocks of magnetic field: Magnetic Resonance Imaging: 1. The magnet which generates the main magnetic field, hardware and denoted by (B0 ). The orientation of the main magnetic excitation David Norris field is by convention along the z-axis. 2. The gradient set, which generates switched magnetic Introductory electromag- field gradients in (Bz ). There are three gradient fields netism giving a gradient in Bz along the x, y, and z-axes. These MRI system components are denoted Gx , Gy , Gz. Excitation 3. The radio-frequency system consisting of: 3.1 The transmitter and resonator which together generate a rotating field known as the (B1 ) field. 3.2 The receiver coil or coils which detect and amplify the weak MR signal. 7 / 31 System components To perform MRI we need three very different forms of The building blocks of magnetic field: Magnetic Resonance Imaging: 1. The magnet which generates the main magnetic field, hardware and denoted by (B0 ). The orientation of the main magnetic excitation David Norris field is by convention along the z-axis. 2. The gradient set, which generates switched magnetic Introductory electromag- field gradients in (Bz ). There are three gradient fields netism giving a gradient in Bz along the x, y, and z-axes. These MRI system components are denoted Gx , Gy , Gz. Excitation 3. The radio-frequency system consisting of: 3.1 The transmitter and resonator which together generate a rotating field known as the (B1 ) field. 3.2 The receiver coil or coils which detect and amplify the weak MR signal. 7 / 31 System components To perform MRI we need three very different forms of The building blocks of magnetic field: Magnetic Resonance Imaging: 1. The magnet which generates the main magnetic field, hardware and denoted by (B0 ). The orientation of the main magnetic excitation David Norris field is by convention along the z-axis. 2. The gradient set, which generates switched magnetic Introductory electromag- field gradients in (Bz ). There are three gradient fields netism giving a gradient in Bz along the x, y, and z-axes. These MRI system components are denoted Gx , Gy , Gz. Excitation 3. The radio-frequency system consisting of: 3.1 The transmitter and resonator which together generate a rotating field known as the (B1 ) field. 3.2 The receiver coil or coils which detect and amplify the weak MR signal. All three of these fields have markedly different characteristics. 7 / 31 Magnets The building blocks of Magnetic Resonance Imaging: hardware and excitation The goal is to have a magnet David Norris with a very homogeneous magnetic field in the region Introductory electromag- where you are imaging. netism An infinite solenoid will do this MRI system components (a finite solenoid is shown). Excitation However, this is not a practical design, and compromises have to be made to restrict the dimensions and allow access. 8 / 31 Magnet systems old and new The building blocks of Magnetic Resonance Imaging: The Aberdeen Mark 1 hardware and imager, circa 1980 excitation David Norris Introductory electromag- netism MRI system components Excitation 9 / 31 Magnet systems old and new The building blocks of Magnetic Resonance Imaging: The Aberdeen Mark 1 A Siemens trio system, hardware and imager, circa 1980 circa 2005 excitation David Norris Introductory electromag- netism MRI system components Excitation 9 / 31 Characteristics of the main magnetic field The building blocks of Magnetic Resonance Imaging: hardware and excitation I This is the strongest of the magnetic fields, with a David Norris strength in Teslas. Introductory electromag- netism MRI system components Excitation 10 / 31 Characteristics of the main magnetic field The building blocks of Magnetic Resonance Imaging: hardware and excitation I This is the strongest of the magnetic fields, with a David Norris strength in Teslas. Introductory electromag- I In a superconducting system the magnet is always on. netism MRI system components Excitation 10 / 31 Characteristics of the main magnetic field The building blocks of Magnetic Resonance Imaging: hardware and excitation I This is the strongest of the magnetic fields, with a David Norris strength in Teslas. Introductory electromag- I In a superconducting system the magnet is always on. netism MRI system I The field should be stable in time. components Excitation 10 / 31 Characteristics of the main magnetic field The building blocks of Magnetic Resonance Imaging: hardware and excitation I This is the strongest of the magnetic fields, with a David Norris strength in Teslas. Introductory electromag- I In a superconducting system the magnet is always on. netism MRI system I The field should be stable in time. components Excitation I It should also be homogeneous over the imaging volume. 10 / 31 On homogeneity and shims The building blocks of Magnetic Resonance Imaging: hardware and I It is impossible in practice to build a magnet with a excitation perfectly homogeneous field. Furthermore the presence David Norris of an object will distort the field. Introductory electromag- netism MRI system components Excitation 11 / 31 On homogeneity and shims The building blocks of Magnetic Resonance Imaging: hardware and I It is impossible in practice to build a magnet with a excitation perfectly homogeneous field. Furthermore the presence David Norris of an object will distort the field. Introductory electromag- I In order to compensate for these imperfections netism MRI system additional weak static fields are applied using room components temperature coils. These are known as shim coils. Excitation 11 / 31 On homogeneity and shims The building blocks of Magnetic Resonance Imaging: hardware and I It is impossible in practice to build a magnet with a excitation perfectly homogeneous field. Furthermore the presence David Norris of an object will distort the field. Introductory electromag- I In order to compensate for these imperfections netism MRI system additional weak static fields are applied using room components temperature coils. These are known as shim coils. Excitation I The current in these coils is adjusted before a measurement to give the best homogeneity possible. 11 / 31 Magnetic field gradients The building blocks of Magnetic I Imaging is made possible by the application of pulsed Resonance Imaging: magnetic field gradients. hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 12 / 31 Magnetic field gradients The building blocks of Magnetic I Imaging is made possible by the application of pulsed Resonance Imaging: magnetic field gradients. hardware and excitation I The axis system is defined so that the main magnetic David Norris field is along the z-axis, the x-axis is left-right for Introductory electromag- someone lying on their backs, and the y-axis is netism front-back. MRI system components Excitation 12 / 31 Magnetic field gradients The building blocks of Magnetic I Imaging is made possible by the application of pulsed Resonance Imaging: magnetic field gradients. hardware and excitation I The axis system is defined so that the main magnetic David Norris field is along the z-axis, the x-axis is left-right for Introductory electromag- someone lying on their backs, and the y-axis is netism front-back. MRI system components I The gradients, produce a small linear variation in the Excitation z-component of the main field as a function of x,y,z. 12 / 31 Magnetic field gradients The building blocks of Magnetic I Imaging is made possible by the application of pulsed Resonance Imaging: magnetic field gradients. hardware and excitation I The axis system is defined so that the main magnetic David Norris field is along the z-axis, the x-axis is left-right for Introductory electromag- someone lying on their backs, and the y-axis is netism front-back. MRI system components I The gradients, produce a small linear variation in the Excitation z-component of the main field as a function of x,y,z. I As the frequency of the MRI signal is proportional to the magnetic field the signal frequency is then linearly proportional to position. 12 / 31 Magnetic field gradients The building blocks of Magnetic I Imaging is made possible by the application of pulsed Resonance Imaging: magnetic field gradients. hardware and excitation I The axis system is defined so that the main magnetic David Norris field is along the z-axis, the x-axis is left-right for Introductory electromag- someone lying on their backs, and the y-axis is netism front-back. MRI system components I The gradients, produce a small linear variation in the Excitation z-component of the main field as a function of x,y,z. I As the frequency of the MRI signal is proportional to the magnetic field the signal frequency is then linearly proportional to position. 12 / 31 Some facts on gradients The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris 1. Current whole body systems reach gradient strengths of about 80 mT m−1. Introductory electromag- netism MRI system components Excitation 13 / 31 Some facts on gradients The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris 1. Current whole body systems reach gradient strengths of about 80 mT m−1. Introductory electromag- netism 2. The maximum field generated by these is about 40 mT MRI system or two orders of magnitude smaller than the main field. components Excitation 13 / 31 Some facts on gradients The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris 1. Current whole body systems reach gradient strengths of about 80 mT m−1. Introductory electromag- netism 2. The maximum field generated by these is about 40 mT MRI system or two orders of magnitude smaller than the main field. components 3. The gradient fields can be switched in about 100 µs. Excitation 13 / 31 Gradient coil arrangement The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 14 / 31 Radiofrequency coils The building blocks of Magnetic Resonance Imaging: 1. The coil used to generate the RF field is often called a hardware and excitation resonator. It can generally both transmit the RF field David Norris and detect the MR signal. Introductory electromag- netism MRI system components Excitation 15 / 31 Radiofrequency coils The building blocks of Magnetic Resonance Imaging: 1. The coil used to generate the RF field is often called a hardware and excitation resonator. It can generally both transmit the RF field David Norris and detect the MR signal. Introductory 2. A common design is the birdcage coil. This generates a electromag- netism weak magnetic field of about 30 µT that rotates about MRI system the long axis of the coil at radiofrequency (MHz range). components The long axis of the coil is parallel to the z-axis of the Excitation magnet. 15 / 31 Radiofrequency coils The building blocks of Magnetic Resonance Imaging: 1. The coil used to generate the RF field is often called a hardware and excitation resonator. It can generally both transmit the RF field David Norris and detect the MR signal. Introductory 2. A common design is the birdcage coil. This generates a electromag- netism weak magnetic field of about 30 µT that rotates about MRI system the long axis of the coil at radiofrequency (MHz range). components The long axis of the coil is parallel to the z-axis of the Excitation magnet. 3. Within the coil the transmission field should be as homogeneous as possible. 15 / 31 Radiofrequency coil: birdcage The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 16 / 31 A practical coil The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 17 / 31 Multi-channel coils The building blocks of Magnetic Resonance Imaging: hardware and excitation In modern systems separate coils are used for transmit and David Norris receive. Multiple channel receiver coils have two Introductory advantages: electromag- netism 1. They improve sensitivity. MRI system components Excitation 18 / 31 Multi-channel coils The building blocks of Magnetic Resonance Imaging: hardware and excitation In modern systems separate coils are used for transmit and David Norris receive. Multiple channel receiver coils have two Introductory advantages: electromag- netism 1. They improve sensitivity. MRI system components 2. They can encode some degree of spatial information. Excitation This can be used to speed up image acquisition. 18 / 31 Prototype multichannel coils The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 19 / 31 Schematic MR system The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 20 / 31 What happens when we lie in the magnet? The building I Some atomic nuclei have a property known as spin. blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 21 / 31 What happens when we lie in the magnet? The building I Some atomic nuclei have a property known as spin. blocks of Magnetic Resonance I As the nuclei are also electrically charged the spin Imaging: hardware and makes them behave like small magnets. This is excitation because the charge rotates in a circle, which is David Norris equivalent to a current loop. Introductory electromag- netism MRI system components Excitation 21 / 31 What happens when we lie in the magnet? The building I Some atomic nuclei have a property known as spin. blocks of Magnetic Resonance I As the nuclei are also electrically charged the spin Imaging: hardware and makes them behave like small magnets. This is excitation because the charge rotates in a circle, which is David Norris equivalent to a current loop. Introductory electromag- netism MRI system components Excitation 21 / 31 Precession of a single nucleus The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris If you put these nuclei in a magnetic field then the magnet Introductory will try to align itself with the field, but the rotation will force it electromag- netism to precess about the field. This is a motion like that of a MRI system child’s spinning top. components Excitation 22 / 31 Precession of a single nucleus The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 23 / 31 Precession of a single nucleus The building blocks of Magnetic Resonance Imaging: hardware and The precession frequency is given by excitation David Norris ω0 = γB0 (1) Introductory electromag- netism Here ω0 is the angular frequency (often called the Larmor MRI system frequency), B0 the main magnetic field and γ the components Excitation gyromagnetic ratio which is nuclear specific. This tells us that the frequency of rotation is always proportional to the magnetic field. 24 / 31 How does bulk magnetisation appear? The building blocks of Magnetic 1. For protons there are two allowed Resonance Imaging: states: precessing parallel or hardware and excitation antiparallel to the main field David Norris (diagram). Introductory electromag- netism MRI system components Excitation 25 / 31 How does bulk magnetisation appear? The building blocks of Magnetic 1. For protons there are two allowed Resonance Imaging: states: precessing parallel or hardware and excitation antiparallel to the main field David Norris (diagram). Introductory 2. There are slightly more protons in electromag- the parallel state because this netism has a lower energy level. MRI system components Excitation 25 / 31 How does bulk magnetisation appear? The building blocks of Magnetic 1. For protons there are two allowed Resonance Imaging: states: precessing parallel or hardware and excitation antiparallel to the main field David Norris (diagram). Introductory 2. There are slightly more protons in electromag- the parallel state because this netism has a lower energy level. MRI system components 3. As the protons are randomly Excitation oriented on the cone the components of magnetisation in the transverse plane cancel. 25 / 31 How does bulk magnetisation appear? The building blocks of Magnetic 1. For protons there are two allowed Resonance Imaging: states: precessing parallel or hardware and excitation antiparallel to the main field David Norris (diagram). Introductory 2. There are slightly more protons in electromag- the parallel state because this netism has a lower energy level. MRI system components 3. As the protons are randomly Excitation oriented on the cone the components of magnetisation in the transverse plane cancel. 4. There is hence only a net magnetisation parallel to the main magnetic field. 25 / 31 So how do we get a signal? The building blocks of I We understand now that if we put a head in a strong Magnetic Resonance magnetic field the water protons (amongst others) will Imaging: hardware and be weakly magnetised. excitation David Norris Introductory electromag- netism MRI system components Excitation 26 / 31 So how do we get a signal? The building blocks of I We understand now that if we put a head in a strong Magnetic Resonance magnetic field the water protons (amongst others) will Imaging: hardware and be weakly magnetised. excitation David Norris I To get a signal from the head we need to tease the magnetisation away from being parallel to the magnetic Introductory electromag- field, because then it will precess about the magnetic netism field. MRI system components Excitation 26 / 31 So how do we get a signal? The building blocks of I We understand now that if we put a head in a strong Magnetic Resonance magnetic field the water protons (amongst others) will Imaging: hardware and be weakly magnetised. excitation David Norris I To get a signal from the head we need to tease the magnetisation away from being parallel to the magnetic Introductory electromag- field, because then it will precess about the magnetic netism field. MRI system components I We can do this by applying an additional magnetic field, Excitation the B1 field. This field is perpendicular to the main magnetic field. It rotates at the Larmor frequency (for protons in a 3T magnet this is 125 million times per second). 26 / 31 So how do we get a signal? The building blocks of I We understand now that if we put a head in a strong Magnetic Resonance magnetic field the water protons (amongst others) will Imaging: hardware and be weakly magnetised. excitation David Norris I To get a signal from the head we need to tease the magnetisation away from being parallel to the magnetic Introductory electromag- field, because then it will precess about the magnetic netism field. MRI system components I We can do this by applying an additional magnetic field, Excitation the B1 field. This field is perpendicular to the main magnetic field. It rotates at the Larmor frequency (for protons in a 3T magnet this is 125 million times per second). I This field is generated by the resonator. 26 / 31 Excitation The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris Introductory electromag- netism MRI system components Excitation 27 / 31 Excitation The building blocks of Magnetic Resonance Imaging: hardware and excitation David Norris The static magnetic field is orange, and remains stationary. Introductory The rotating B1 -field is yellow. Because this rotates at the electromag- netism Larmor frequency the angle with the magnetisation (white) is MRI system components fixed at 90°. Excitation 28 / 31 Excitation 1. If we rotate with the radiofrequency field then the picture is simplified. 1. If we rotate with the radiofrequency field then the picture is simplified. 2. The application of a radio-frequency field can rotate the magnetisation about any angle. 1. If we rotate with the radiofrequency field then the picture is simplified. 2. The application of a radio-frequency field can rotate the magnetisation about any angle. 3. A 90° pulse generates the maximum amount of transverse magnetisation and hence the maximum signal. 1. If we rotate with the radiofrequency field then the picture is simplified. 2. The application of a radio-frequency field can rotate the magnetisation about any angle. 3. A 90° pulse generates the maximum amount of transverse magnetisation and hence the maximum signal. 4. The RF pulse typically lasts a couple of milliseconds, and is then turned off. Signal can not be acquired whilst the pulse is being transmitted. 1. If we rotate with the radiofrequency field then the picture is simplified. 2. The application of a radio-frequency field can rotate the magnetisation about any angle. 3. A 90° pulse generates the maximum amount of transverse magnetisation and hence the maximum signal. 4. The RF pulse typically lasts a couple of milliseconds, and is then turned off. Signal can not be acquired whilst the pulse is being transmitted. 5. Excitation is most effective at the Larmor frequency.  The building blocks of Magnetic Resonance Imaging: I The rotating hardware and excitation magnetisation induces a David Norris voltage according to Faraday’s law. Introductory electromag- netism MRI system components Excitation 31 / 31  The building blocks of Magnetic Resonance Imaging: I The rotating hardware and excitation magnetisation induces a David Norris voltage according to Faraday’s law. Introductory electromag- netism I Do not imagine one MRI system large magnet in the components middle of the object, but Excitation rather a distribution of small objects. 31 / 31  The building blocks of Magnetic Resonance Imaging: I The rotating hardware and excitation magnetisation induces a David Norris voltage according to Faraday’s law. Introductory electromag- netism I Do not imagine one MRI system large magnet in the components middle of the object, but Excitation rather a distribution of small objects. I Essential behaviour is like a bicycle dynamo. 31 / 31  The building blocks of Magnetic Resonance Imaging: I The rotating hardware and excitation magnetisation induces a David Norris voltage according to Faraday’s law. Introductory electromag- netism I Do not imagine one MRI system large magnet in the components middle of the object, but Excitation rather a distribution of small objects. I Essential behaviour is like a bicycle dynamo. 31 / 31