Introduction to Magnetic Resonance Imaging

Transcription

1 Introduction to Magnetic Resonance Imaging MRI of the brain, ca ca ca Modality Characteristics and Comparison Radiography CT scanning Nuclear medicine MRI transmission modalities emission modality use ionizing radiation use electromagnetic energy a small portion of the spectrum is useful for medical imaging Ultrasound uses sound waves 1-20MHz reflection modality 1

2 Is MRI Dangerous? No. Is MRI Dangerous? MAYBE A LITTLE How urban legends begin.. : Some Kidney Patients Suffer MRI Poisoning A growing number of people are becoming afflicted with an incurable, manmade disease that is related to a common medical procedure performed every single day in this country, a KCRA 3 investigation has found. MSNBC Note that this was NOT MRI poisoning this was gadolinium poisoning, an occasionally-used MR contrast agent 2

3 Very briefly how it works courtesy Joe Hornak, RIT Very briefly how it works courtesy Joe Hornak, RIT The very strong magnet with gradient coils inside 3

4 Very briefly how it works courtesy Joe Hornak, RIT Introduction to MRI: Spin Microscopic view Macroscopic view RF excitation how do we turn spin into detectable signal? Relaxation what makes the detectable signal decay? Spin echoes how can we overcome this decay? Contrast how can we use relaxation to tell us about the properties of our sample? Gradients & signal localization Frequency encoding Slice selection Readout Image formation Nuclear Magnetic Resonance NMR Magnetic Resonance Imaging MRI 4

5 Spin microscopic view: Protons have a small electric (positive) charge They spin => produce small magnetic field ( spin ) Under normal conditions, the spins are randomly oriented Water is the largest source of protons in the body (hydrogen nuclei), followed by fat Nuclei that have an odd number of protons have net spin, or a magnetic moment, and are candidates for NMR Fortunately, hydrogen has one proton and thus a net spin that we can influence Spin microscopic view: Just like a compass aligning with the earth s field, a spinning proton placed within a large external magnetic field, B 0, will align with or against the field. A slight excess will align with the field The net result is an alignment with the external field Notes that will affect our macroscopic view later: a photon at the correct frequency for the magnetic field can cause the individual nuclei to change their direction of alignment (to flip) Reality of alignment : 5

6 Spin The protons aligned with the field are in a lower energy state than those aligned against the field The larger the external B0 field, the greater the difference in energy levels The larger the external B0 field, the larger the excess number of protons aligned with the field. Distribution of 2 million protons at different field strengths Spin macroscopic view Consider: how many excess protons are there in a single voxel of water at 1.5T? Assume a voxel is 2 x 2 x 5mm = 0.02ml Avogadro s Number says 6.02 x 1023 molecules/mole Facts: 1 mole of water weighs 18 grams (16g of O and 2g of H), has 2 moles of H, and fills 18ml So 1 voxel of water has 2 moles H molecules H 0.02 ml/voxel total protons mole water mole H 18 ml/mole From previous slide, excess protons in the low-energy state at 1.5T are 9/2million=> or 6 million billion Point: we can ignore the microscopic (quantum) view and focus on the macroscopic (classical) view 6

7 Spin & Larmor frequency macroscopic view The total number of excess spins is called M0, the net magnetization, and it is aligned along the main field, B0 M0 in a magnetic field, B0, acts like a dreidle in the presence of gravity and precesses about the field at a frequency proportional to B0 f0=γb0, the Larmor Frequency where γ, gamma, is the gyromagnetic ratio = MHz/Tesla for protons in hydrogen RF excitation getting a detectable signal When you input a magnetic field, B1, at the Larmor frequency (an RF field generated by an RF coil ), you can tip the magnetization vector into the x-y plane, or transverse plane Rotating frame A note on the Rotating Frame A frame of reference that is rotating at the Larmor frequency. i.e. x and y axes are rotating at f0 and z =z. The magnetization continues to precess at the Larmor frequency in the transverse plane and this moving magnetic field can be detected by a pickup coil. Transverse magnetization can be detected D Ădž ƐŝŐŶĂů ǁ ŝƚś ŵădž ƚƌăŷɛǀ ĞƌƐĞ ŵăőŷğɵnjăɵžŷ ї α=90o Longitudinal magnetization is not detected with the RF coil 7

8 Relaxation the decay of the detectable signal T1 relaxation, or longitudinal relaxation: The z component recovers, decreasing the net transverse (detectable) component: T2 relaxation, or transverse relaxation, or spin-spin relaxation: The magnetization vector is, in reality, composed of many many spins that are not all exposed to the exact same magnetic field due to purely random spin-spin interactions => they all precess at slightly different frequencies. This is called dephasing. Relaxation the decay of the detectable signal Note: usually T2 is so much shorter than T1 that we consider the signal decay to be due primarily to T2 effects. We consider T1 to be good since that is how we grow back our magnetization to use again. When the transmitter is turned off, the protons immediately begin to reradiate the absorbed energy from the RF pulse that tipped them. T1 - Growth of longitudinal magnetization FID Free Induction Decay: The signal unaffected by any gradient. Contains no positional information 8

9 Relaxation the decay of the detectable signal Reality: T2* relaxation: Not only do the spins dephase because of spin-spin interactions, but also because of (repeatable, nonrandom, fixed) imperfections in the magnetic field Spin Echoes overcoming (some of) the decay The effects of the non-random, repeatable, parts of T2* decay can be overcome A note on gradient echoes: instead of using the 180o RF pulse to achieve rephasing (an echo), a gradient echo is achieved by forcing dephasing of the spins by deliberately placing a change in the magnetic field (a gradient ) across them for a time, and then reversing the polarity of that gradient to force rephasing. This is a fast way to create an echo, and works well in homogenous main fields (where the repeatable, non-random parts of T2* decay we discussed are small). You will be using spin echoes. 9

10 E D Z ї D Z/ ƐŝŐŶĂů ůžđăůŝnjăɵžŷ So far, our signal (FID or echo) is from the entire sample Recall: the frequency of the signal we receive is a function of the strength of the magnetic field Therefore: if we can control the strength of the magnetic field in space, we will receive back a signal whose frequency content is spatially dependent. (then we just need an IFT) We need gradients in our main magnetic field E D Z ї D Z/ ƐŝŐŶĂů ůžđăůŝnjăɵžŷ The gradient coils do not change the direction of the main magnetic field. They add or subtract from the magnitude of the main field. The amount they add or subtract is a function of spatial position. B ( B0 Gx x Gy y Gz z ) zˆ X -gradient z -gradient Y -gradient 10

11 NMR MRI : frequency encoding - slice selection Place a gradient in the magnetic field that yields a frequency distribution across the sample that is a function of position in z: If a pure sinusoidal RF excitation at a specific frequency were applied, only an infinitesimally thin slice of the body would undergo forced precession, or tipping, and yield signal. This is not feasible (or desirable) in practice. Actually, create a waveform that has a range of frequencies (frequency content or spectrum or bandwidth) and excites a slice. Slice (along z) B0 Bandwidth of RF pulse NMR MRI : frequency encoding - readout Turn on a readout gradient during the acquisition (or readout/digitization) of the signal, and the signal will have frequency content that is a function of x. i.e. the signal is spatially encoded in the x direction. y object: Gradient strength 0 weak strong x FID FT spectrum projection f f f 11

12 NMR MRI : image formation Now consider doing the same thing with a gradient along y: y object: x f FT spectrum projection FID For you to form an image, you could acquire many of these projections and then reconstruct NMR MRI : image formation Np Phase Encode Repetitions Typically, to create an MR image, you need to fill 2D k -space with raw data the repeated echoes you collect ky Image domain k-space domain 2D-FFT Nf samples kx 12

13 NMR MRI : image formation How long does it take to fill k-space, or to create a raw data set? It depends on how long we wait between pulses of RF. This is our TR, or repetition time. Longer TR more time for the longitudinal magnetization to grow back according to T1 more magnetization to tip into the transverse plane more signal By controlling our TR and TE (echo time time we wait before digitizing our echo), we can control our image contrast. T1W TSE with CLEAR 0.5 x 0.6 x 4.0 mm 13 slices in 5:04 min T2W TSE with CLEAR SENSE Spine coil 0.5 x 0.6 x 4.0 mm 13 slices in 5:22 min Courtesy: Kyunghee University Hospital, Korea STIR TSE with CLEAR 0.5 x 0.9 x 4.0mm 13 slices in 9:58 min 13

14 Ultra high resolution PDW TSE Wrist using a-tse and SENSE to increase resolution and reduce scantime x 0.15 x 2.0 mm, 16 slices in 6:14 min Ultra high resolution PDW TSE Foot using a-tse and SENSE to increase resolution and reduce scantime x 0.2 x 2.5mm, 18 slices in 7:16 mins 14

15 Whole Body Angiography in 4 stations using the integrated body coil Optimal resolution per station using MobiFlex, scan time 1:10 min Courtesy: Catharina Hospital Eindhoven, The Netherlands Hi-Res T2W TSE Whole Body Imaging using the integrated body coil 0.5 x 0.5 x 6.0 mm 12 slices, 2:27 min / station FiberTrak of relevant tracts in a patient with large pathology 15 directions DTI using SENSE 2.0 x 2.0 x 2.0 mm 60 slices in 5:13 min Courtesy: University of Michigan, Ann Arbor, MI, USA 15

16 High resolution Time of Flight of the Circle of Willis 0.2 x 0.2 x 0.5 mm, 148 slices High resolution Venous BOLD using SENSE 0.5 x 0.5 x 0.5 mm, 200 slices 16

17 2D CSI (TE 144 ms) using SENSE to reduce scantime, 4:32 min Cho / Cr / NAA / Lip ratios differ in lesions compared to normal tissue Courtesy: University of Michigan, Ann Arbor, MI, USA IViewBold processing package for functional studies Motor task experiment on a patient with a large meningioma Courtesy: Erasme University Hospital, Brussels, Belgium 17

18 Quantitative Flow in a breath hold with color overlay 2.0 x 1.3 x 8.0 mm, 40 phases, 16 sec Where is MRI going? MR is diversifyingdedicated systems for head, cardiac, extremities Open systems for MR guided surgery already exist HIGH FIELD METABOLIC MEDICINE 18

19 Cost-Reduction Through Technology Low-cost, dedicated MRI units are now on the market, and are being developed for many applications Other technologies in future could lead to further reductions Prepolarized MRI High-Tc superconductors MRI at A&M 4.7T 40cm geeks 4.7T 33cm 19

20 Evolution of MRI Nuclear Magnetic Resonance Purcell Lauterbur Spatial localization Analytical NMR Mansfield Human MRI Lab-on-a-chip Bench-tobedside MRI analysis` Bloch Damadian Relaxation times Ernst 2D localization Magnetic Resonance Imaging Proton Density T1 weighting T2 weighting DCE MRI Chemical Separation FATSAT CSI 1978 Flow MRA 1984 Clinical MRI at 1.5T => SNR Real Time MRI 1988 BOLD fmri Fast Gradients 1992 High Fields & Parallel MRI quantitative diagnostic clinical MRI clinical/ anatomical/ hospital MRI 2004 MRI at A&M 20

21 MRI at A&M In vivo Experiment Setting Scavenging filter Loading coil/animal holder Lizard Lamp Warmed Knockdown chamber Nosecone Nonmagnetic isoflurane Heated evaporator surgical bed Water pump and heater Simulator Scavenging filter Respiratory Main Isoflurane Nonmagnetic oxygen tank module control evaporator module ECG/temp module Control heater Fan 19 MRI at A&M A C B D Fig 2: MR images of the head region of the rat obtained with a 4.7Tesla/33 cm diameter scanner interfaced to a Varian Inova console. Shown here are axial scans of 4 slices (1.5mm) taken after injection of Gd-DTPA contrast agent. The imaging parameters were: TR = 500 msec, TE = 8 msec, 2 averages, field of view: 50mm x 50 mm, resolution: 256x256, imaging time: 4 min and 22 secs. Animals were anesthetized with injectable anesthesia and contrast dye was injected via an in-dwelling tail vein catheter. 21

22 ISBI & ISMRM 2013 abstracts: U Mich (3T) and UTSW (7T) collaborations Figure 3: Eight gradient echo images acquired on a GE 3T clinical scanner with loop array transmit coil. Only one channel was used to transmit for each images Inter-element coupling is well suppressed. Courses ECEN 411 Introduction to MRI Lab (3) ECEN 648 Principles of MR Imaging (3) ECEN/BMEN 427/627 MR Engineering Lab (3) ECEN 617 Advanced Signal Processing for MRI BMEN 489/689 Biomedical Electromagnetics 22