2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 1 Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework** Consortium leader PETER PAZMANY CATHOLIC UNIVERSITY Consortium members SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund *** **Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben ***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg. PETER PAZMANY CATHOLIC UNIVERSITY SEMMELWEIS UNIVERSITY sote_logo.jpg dk_fejlec.gif INFOBLOKK 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 2 Peter Pazmany Catholic University Faculty of Information Technology BIOMEDICAL IMAGING MAGNETIC RESONANCE IMAGING (MRI) -BASICS www.itk.ppke.hu (Orvosbiológiai képalkotás ) (Mágneses Rezonancia Képalkotás (MRI) -Bevezetés) ISTVÁN KÓBOR, GYÖRGY ERŐSS 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 3 www.itk.ppke.hu 180px-MR_Knee image_mini(1) 200px-Mra1 Biomedical Imaging: Magnetic Resonance Imaging -Basics MR Images 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 4 www.itk.ppke.hu Tesla and Gauss are measures of magnetic field strength – Earth’s magnetic field ~0.5 Gauss. – 1Tesla = 10,000 Gauss. – Our fMRIsystem is 3T. ~x60,000 earth’s field strength magnet.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics TÁMOP –4.1.2-08/2/A/KMR-2009-0006 5 www.itk.ppke.hu • Outside magnetic field:– Spins randomly oriented • In magnetic field:– Spins tend to align parallel or anti-parallel to magnetic field – At room temperature, ~4 parts per million more protons per Tesla align with versus against field – As field strength increases, there is a bigger energy difference between parallel and anti-parallel alignment (faster rotation = more energy) – A larger proportion will align parallel to field – More energy will be released as nuclei align – Therefore, MR signal increases with square of field strength 2011.10.04.. Biomedical Imaging: Magnetic Resonance Imaging -Basics Signal and Field Strength 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 6 www.itk.ppke.hu Signal and Field Strength – Most clinical MRI: 1.5T – fMRI systems: 3.0T – Maximum for NbTi MRI ~11.7T – Field strength influences:• Faster Larmor frequency • Bigger energy difference between parallel and anti-parallel alignment– Larger ratio of nuclei aligned = more signal – More signal as nuclei realign • Reduced TR and TE: less time to take images Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 7 www.itk.ppke.hu • In theory:– Signal increases with square of field strength – Noise increases linearly with field strength – A 3T scanner should have twice SNR of 1.5T scanner; 7T should have ~4.7 times SNR of 1.5T • Unfortunately, physiological artifacts also increase, so advantage is less in practice • Benefits: speed, resolution • Costs: artifacts, money, wavelength effects, auditory noise Biomedical Imaging: Magnetic Resonance Imaging -Basics Signal and Field Strength 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 8 www.itk.ppke.hu • MRI signals are in the same range as FM radio and TV (30-300MHz) • MRI frequency is non-ionizing radiation, unlike X-rays • Absorbed RF will cause heating • Specific absorption rate (SAR): measure of the energy absorbed by tissue– Increases ~ with square of field strength – Higher SAR = more energy = more signal = more heating – FDA limits SAR, and is a limiting factor for some protocols (3 W/kg averaged over 10 minutes) Electromagnetic Spectrum Biomedical Imaging: Magnetic Resonance Imaging -Basics Power, telephone FM radio, TV MR 1 THz X-rays Repetitive Visual stimulation 1 GHz 1 MHz microwaves 1015Hz infrared UV 1018Hz gamma 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 9 www.itk.ppke.hu MRI terminology • Orientation: typically coronal, sagittalor axial, can be in-between these (oblique) • Matrix Size:– Voxelsin each dimension • Field of view:– Spatial extent of each dimension • Resolution:– FOV/Matrix size scanner.jpg Philips Achieva 3T Scanner Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 10 •MRImagnetic resonance imaging images of biological tissues,structural studies– static magnetic field + a series of changing magnetic fields and oscillating electromagnetic fields (pulse sequence) – depending on frequency of electromagnetic fields, energy is absorbed by hydrogen nuclei (excitation) – later the energy is emitted by the nuclei – the amount of energy depends on numbers and types of nuclei present • Advantages of MRI – No ionizing radiation exposure – Better spatial resolutionthanCT • Disadvantages– No ferrous metal! www.itk.ppke.hu Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 11 History of MR/ MRI/ fMRI: NMR = nuclear magnetic resonance – Felix Block and Edward Purcell• 1946: atomic nuclei absorb and re-emit radio frequency energy • 1952: Nobel prize in physics– nuclear: properties of nuclei of atoms – magnetic: magnetic field required – resonance: interaction between magnetic field and radio frequency www.itk.ppke.hu felix_bloch_photo_nobelpage Edward_purcell_photo_nobelpage Felix Bloch Edward Purcell C:\Documents and Settings\kjlk\Desktop\precess.png Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 12 History of MR/ MRI/ fMRI: -1971: MRI Tumor detection (Damadian) -1973: Lauterbur suggests NMR could be used to form images -1977: clinical MRI scanner patented -1977: Mansfield proposes echo-planar imaging (EPI) to acquire images faster -2003: Nobel prize was awarded to Paul Lauterbur and Sir Peter Mansfield (excluding Damadian –huge controversy) fMRI -1990: Ogawa observes BOLD effect with T2* blood vessels became more visible as blood oxygen decreased -1991: Belliveau observes first functional images using a contrast agent -1992: Ogawa et al. and Kwong et al. publish first functional images using BOLD signal www.itk.ppke.hu Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 13 www.itk.ppke.hu The First ZMR NMR Image Lauterbur, P.C. (1973). Image formation by induced local interaction: Examples employing nuclear magnetic resonance. Nature, 242, 190-191. Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 14 www.itk.ppke.hu Damadian_Notebook_1_Mattson damadian_first_image Early Human MR Images (Damadian) Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 15 www.itk.ppke.hu Damadian_First_Image_Mattson Mink5 Image –Damadian(1977) Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 16 www.itk.ppke.hu The firstPhilips MR, 1978 (0,15T) first_philipsMR.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 17 www.itk.ppke.hu The firstSiemens MR, 1980 (0,2T) siemens_scanner.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 18 www.itk.ppke.hu 030805-056-km 040610-08-km Typical 1.5/3.0T MR system Special Open-MR system Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 19 Nuclear spins A nucleus of hydrogen – consists of one proton – carries a positive charge – rotates around its axis because of thermal energy electrical current and magnetic source spin www.itk.ppke.hu proton1.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 20 •Nuclei line up with magnetic moments either in a parallel (lower energy level) or anti-parallel configuration(higher energy level). • In body tissues more line up in parallel creating a small additional magnetization Min the direction of B0. www.itk.ppke.hu Nuclei spin axis not parallel to B0 field direction. Nuclear magnetic moments precessabout B0. B0 Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 21 www.itk.ppke.hu Absorption and Relaxation •Our RF transmission is absorbed by atoms at Larmorfrequency • After the RF pulse, atoms will begin to realign with the magnetic field: relaxation • During this period, an RF signal is emitted • This signal will be at the Larmorfrequency • An antenna can measure this signal Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 22 www.itk.ppke.hu •Frequency of precession of magnetic moments given by Larmorrelationship f = .x B0 f = Larmorfrequency (mHz) g= Gyromagneticratio (mHz/Tesla) B0= Magnetic field strength (Tesla) larmour.jpg g~ 43 mHz/Tesla Larmorfrequencies of RICs MRIs 3T ~ 130 mHZ 7T ~ 300 mHz 11.7T ~ 500 mHz B0 Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 23 www.itk.ppke.hu Radiofrequency Pulses •A radiofrequency (RF) pulse at the Larmorfrequency will be absorbed • This higher energy state tips the spin, so it is no longer aligned to the field • An RF pulse at any other frequency will not influence the nuclei, onlyresonancefrequency Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 24 www.itk.ppke.hu Hydrogen is the mainstay for MRI •We will focus on Hydrogen– Hydrogen abundant in body (63% of atoms) – Elements with even numbers of neutrons and protons have no spin, so we can not image them (4He, 12C) – 23Na and 31P are relatively abundant, so can be imaged • Larmorfrequency varies for elements:– 1H = 42.58 Mhz/T – 13C = 10.7 Mhz/T – 19F = 40.1 Mhz/T – 31P = 17.7 Mhz/T • Therefore, by sending in a RF pulse at a specific frequency we can selectively energize hydrogen mractivenucl.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 25 www.itk.ppke.hu •Mis parallel to B0since transverse components of magnetic moments are randomly oriented • The difference between the numbers of protons in the parallel and anti-parallel states leads to the net magnetization(M) • Proton density relates to the number of parallel states per unit volume • Signal producing capability depends on proton density netmagnet.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 26 www.itk.ppke.hu duration strength RF Pulse Frequency of rotation of Mabout B1determined by the magnitude (strength) of B1 RF pulse duration and strength determine flip angle flipangle.jpg rf_concept1.jpg rf_concept3.jpg rf_concept4.jpg RF Pulse Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 27 www.itk.ppke.hu •90°RF pulse rotates M into transverse (x-y) plane • Rotation of M within transverse plane induces signalin receiver coil at Larmor frequency • Magnitude signaldependent on proton density and Mxy FID = Free Induction Decay •FID magnitude decays in an exponential manner with a time constant T2. Decay due to spin-spin relaxation fid3.jpg fid.jpg fid2.jpg transversalnetmagnet.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 28 www.itk.ppke.hu •T1-Relaxation: Recovery– Recovery of longitudinal orientation of Malong z-axis – ‘T1 time’ refers to time interval for 63% recovery of longitudinal magnetization – Spin-Lattice interactions • T2-Relaxation: Dephasing– Loss of transverse magnetization Mxy – ‘T2 time’ refers to time interval for 37% loss of original transverse magnetization – Spin-spin interactions,and more t1relax.jpg t2relax.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 29 www.itk.ppke.hu •T1 is shorter in fat (large molecules) and longer in cerebrospinal fluid (CSF) (small molecules). T1 contrast is higher for lower TRs • T2 is shorter in fat and longer in CSF. Signal contrast increased with TE •TR determines T1 contrast • TE determines T2 contrast Image-0050 Image-0051 Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 30 www.itk.ppke.hu Properties of Body Tissues T1 values for B0~ 1Tesla. T2 ~ 1/10thT1 for soft tissues Grey Matter 950 100 White Matter 600 80 Fat 250 60 Blood 1200 100-200 CerebrospinalFluid 4500 2200 Muscle 900 50 T1 (msec) T2 (msec) Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 31 www.itk.ppke.hu T1/T2 weighted images: t1t2.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 32 www.itk.ppke.hu kepkontarsztok.jpg Contrast, Imaging Parameters: • Short TEs reduce T2W • Long TRs reduce T1W Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 33 www.itk.ppke.hu Making a spatial image •To create spatial images, we need a way to cause different locations in the scanner to generate different signals • To do this, we apply gradients • Gradients make the magnetic field slightly stronger at one location compared to another • Lauterbur: first MRI: 2003 Nobel Prize C:\Documents and Settings\kjlk\Desktop\lauterbur.jpg Lauterbur Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 34 www.itk.ppke.hu Slice Selection Gradient •Gradients make field stronger at one location compared to another • Larmorfrequency different along this dimension • RF pulse only energizes slice where field strength matches Larmorfrequency •Gradual slice selection gradients will select thick slices, while steep gradients select thinner slices– The strength of your scanner’s gradients can limit minimum slice thickness – FDA limits speed of gradient shift (dB/dt) and some of our protocols can elicit slight tingling sensation or brief muscle twitches • Position of gradient determines which 2D slice is selected slicegradient.jpg gradient.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 35 www.itk.ppke.hu •Phase encoding gradient: –Orthogonal gradient applied between RF pulse and readout – This adjusts the phase along this dimension – Analogy: Phase encoding is like timezones. Clocks in different zones will have different phases PhaseEncoding spatialencoding.jpg spatialencoding2.jpg spatialencoding5.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 36 www.itk.ppke.hu FrequencyEncoding spatialencoding4.jpg spatialencoding5.jpg spatialencoding6.jpg • Frequency encoding gradient: – Apply final orthogonal gradient when we wish to acquire image – Slice will emit signal at Lamourfrequency, e.g. lines at higher fields will have higher frequency signals – Aka ‘Readout gradient’ Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 37 www.itk.ppke.hu RawMRI image:k-space (frequency domain)ak-space domain image is formed using frequency and phase encoding kspace2.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 38 www.itk.ppke.hu Reconstruction •Medical scanners automatically reconstruct your data • You can manually reconstruct data • Fourier Transforms are slow: 1021-sample data requires >2 million multiplications (2*N2) • Fast Fourier Transform: 1024-sample data requires 20,000 multiplications (2(N log N))– Optimal when data is power of two (64,128,256, 512), reverts to traditional Fourier for prime numbers – This is why most image matrices are a power of 2 Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 39 www.itk.ppke.hu MRI task is to acquire k-space image then transform to a spatial-domain image. kxis sampled (read out) in real time to give N samples. kyis adjusted before each readout MR image is the magnitude of the Fourier transform of the k-space image kspace.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 40 www.itk.ppke.hu The k-space Trajectory •Equations that govern 2D k-space trajectory The kx, kyfrequency coordinates are established by durations(t) and strengthof gradients (G) if Gxis constantkx= gGxt Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 41 www.itk.ppke.hu spinecho_a.jpg spinecho_b.jpg Primary types of Pulse sequences • Spin Echo(SE): Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 42 www.itk.ppke.hu TE = 20 ms e20250 e40250 Spin-EchoImage e60250 e80250 e20500 e40500 e60500 e80500 e201000 e401000 e601000 e801000 TR =250 ms TR =500 ms TR =1000 ms TE = 50 ms TE = 75 ms TE = 100 ms Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 43 www.itk.ppke.hu •Fast Spin Echo (FSE): ffe1.jpg ffe2.jpg ffe3.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 44 www.itk.ppke.hu FastSpin Echo(FSE) Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 45 www.itk.ppke.hu •GradientEcho(GRE): gradecho2.jpg • Fast Low Angle Shot (FLASH): Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 46 www.itk.ppke.hu •EchoPlanarImaging(EPI): epi0.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 47 www.itk.ppke.hu epi.jpg epi3.jpg epi4.jpg EPI Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 48 www.itk.ppke.hu MRI equipment schematics shieldingmain magnet gradient coils RF coils p a t i e n t patient supportRF coils gradient coils magnet controllerRF source gradient amplifierRF receiverRF amplifier A/D convertgradient pulseRF pulse forming forming Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 49 www.itk.ppke.hu Sötét, átlós felfelé Vízszintes szaggatott 10%-os Vízszintes szaggatott Vacuum superconductive coils liquid nitrogen (.770K) liquid helium (.00K) examination field support 180px-Mri_scanner_schematic_labelled Typical structure of an MR superconductivemagnet bore Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 50 www.itk.ppke.hu 400px-Lauterburfig1 Gradientcoils Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 51 www.itk.ppke.hu mri9-3 mri9-4 mri9-5 mri9-10 Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 52 www.itk.ppke.hu assorted%20surf%20coils%202 MRI3axesgrad Gradient coil system „naked” receiver coils without cover Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 53 www.itk.ppke.hu Schoulder coil 030814-243-km wrist2 SKnee FlexM FlexL Breast biopsy SPV FlexS SENSE Head SENSE NV SENSE Spine SENSE Cardiac SENSE Breast SENSE Knee SENSE Shoulder SENSE Wrist SENSE Flex L SENSE Flex M SENSE XL Torso SENSE PV SENSE Flex S Ankle SENSE FootAnkle 030%20pediatric%20lat Syn Pediatric Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 54 www.itk.ppke.hu 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 54 400px-Mrifig2 Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 55 www.itk.ppke.hu MR safety: •Projectile Effects: External • Projectile Effects: Internal • Acoustic Noise • Radiofrequency Energy • Gradient field changes • Claustrophobia •Anyone with implanted metal should see a doctor before going to the scanner– Pacemaker, cochlear implant, shunt, clip, etc. – Dental work and piercings are fine Scanner visit http://www.simplyphysics.com/flying_objects/chair.jpg Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 56 www.itk.ppke.hu Projectile Effects: Internal • Motion of implanted medical devices– Clips, shunts, valves, etc. • Motion or rotation of debris, shrapnel, filings– Primary risk: Metal fragments in eyes • Swelling/irritation of skin due to motion of iron oxides in tattoo and makeup pigments Acoustic noise: • Potential problem with all scans– Short-term and long-term effects • Sound level • OSHA maximum exposure guidelines– 2-4 hours per day • Earplugs reduce these values by 14-29 dB, depending upon fit Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 57 www.itk.ppke.hu Radiofrequency Energy Tissue Heating – Specific Absorption Rate (SAR; W/kg)• Pulse sequences are limited to cause less than a one-degree rise in core body temperature • Scanners can be operated at up to 4 W/kg (with large safety margin) for normal subjects, 1.5 W/kg for compromised patients (infants, fetuses, cardiac) – Weight of subject critical for SAR calculations Burns – Looped wires can act as RF antennas and focus energy in a small area• Most common problem: ECG leads • Necklaces, earrings, piercings, pulse oximeters, any other cabling Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 58 www.itk.ppke.hu Claustrofobia: Most common subject problem – About 10% of patients Ameliorated with comfort measures – Talking with subject – Air flow through scanner – Panic button – Slow entry into scanner Gradientfieldchanges: Peripheral nerve stimulation – May range from distracting to painful – Risk greatly increased by conductive loops• Arms clasped • Legs crossed Theoretical risk of cardiac stimulation – No evidence for effects at gradient strengths used in MRI Biomedical Imaging: Magnetic Resonance Imaging -Basics 2011.10.04.. TÁMOP –4.1.2-08/2/A/KMR-2009-0006 59 www.itk.ppke.hu FDA MRI Guidelines Bo Adults, Children, and Infants age > 1 month 8 T neonates (infants age < 1 month) 4 T dB/dt No discomfort, pain, or nerve stimulation SAR Specific Absorption Rate whole body, average, over >15 min 4 W/Kg head, average, over >10 min 3 W/Kg head or torso, per g of tissue, in >5 min 8 W/Kg extremities, per g of tissue, in >5 min 12 W/Kg AcousticLevel Peak unweighted 140 dB A-weighted rms with hearing protection 99 dBA Biomedical Imaging: Magnetic Resonance Imaging -Basics