MRI Basics and Interpretation PDF

BASICS OF MRI & MRI INTERPRETATION Dr. SONALI VEDRAJ SHARMA WHAT IS MRI? — Produces very clear, detailed pictures of the organs and structures in the body — It is a form of medical imaging that uses no Ionizing radiation — MRI makes use of the property of Nucleus to behave as a tiny magnet when pl...

BASICS OF MRI & MRI INTERPRETATION Dr. SONALI VEDRAJ SHARMA WHAT IS MRI? — Produces very clear, detailed pictures of the organs and structures in the body — It is a form of medical imaging that uses no Ionizing radiation — MRI makes use of the property of Nucleus to behave as a tiny magnet when placed in a large magnetic field due to its proton content. GLOSSARY • MRI: ↑ Magnetic Resonance Imaging. • B0: ↑ The MRI scanner’s main magnetic field. • Precessional Frequency: ↑ The rate at which protons spin in a magnetic field. • RF: ↑ Radio frequency pulse used to tip on resonance protons away from the B0 field. • On Resonance: ↑ Have the same frequency. • Fourier Transform: ↑ A mathematical calculation that is used to change the electrical current in a coil into an image. HISTORY • The first MR image was published in 1973 • The first studies performed on humans were published in 1977 • In 2003, The Nobel Prize in Physiology or Medicine was awarded to Paul C Lauterbeur and Peter Mansfield • Made new MR imaging techniques • Faster and more efficient COMMON USES • Physicians use the MR examination to help diagnose or monitor treatment for conditions such as: • Tumors and other cancer related abnormalities. • Certain types of heart problems. • Blockages or enlargements of blood vessels • Diseases of the liver, such as cirrhosis, and that of other abdominal organs. • Diseases of the small intestine, colon, and rectum SOFT TISSUE ASSESSMENT HOW DOES IT WORK? — An MRI machine uses a powerful magnetic field to align the magnetization of some atoms in the body. — radio frequency fields systematically alter the alignment of this magnetization — This causes the nuclei to produce a rotating magnetic field detectable by the scanner — This information is recorded to construct an image of the body. THE “MAGNET” IN MAGNETIC RESONANCE IMAGING • The MRI scanner is essentially a giant magnet. • The strength of the magnet is measured in a unit called Tesla (T). • Most MRI scanners used in hospitals and medical research clinics are 0.5T, 2T or 3 T = 30,000 gauss. • Putting that in to perspective, the earth’s magnetic field is around 0.5 gauss= 0.00005 T. • A 3T MRI scanner is around 50,000 times stronger than the earth’s magnetic field! WHAT DOES IT CHECK? ØMRI uses magnetic fields and radio waves to measures how much water is in different tissues of the body, maps the location of the water and then uses this information to generate a detailed image. ØThe images are so detailed because our bodies are made up of around 65% water, so we have lots of signal to measure. ØThe water molecule (H2O) is made up of two hydrogen atoms and one oxygen atom. ØThe hydrogen (H) atoms are the part that makes water interesting for MRI, and what we use to measure the signal from the body when we do an MRI scan. A- HYDROGEN PROTON SPINNING ON ITS OWN AXIS B-PROTONS IN OUR BODY WITHOUT MAGNETIC FIELD C- PROTONS IN OUR BODY WITH MAGNETIC FIELD (MRI SCANNER) D- RADIOFREQUENCY PULSE MATCHES PROTON’S PRECESSIONAL FREQUENCY HOW DOES IT WORK • When placed in an MRI scanner, protons’ axes realign them with the scanner’s main magnetic field (BO) • Some will align “up” (parallel) and some will align “down” (anti-parallel), while still spinning around on their own axes. HOW DOES IT WORK • The magnetic fields cancel each other out, to leave only the magnetic field from the small proportion of extra “up” protons, and it is this small magnetic field that we measure using MRI. • The BO field not only affects the hydrogen proton’s alignment, but also affects how fast these protons spin (called precessional frequency/ Larmor Frequency: the rate at which protons spin in a magnetic field.) • This frequency depends on the strength of the magnetic field; the stronger the magnetic field, the faster they spin. HOW DO WE DETECT THE MAGNETIC FIELD? • A Radio frequency pulse (RF pulse) is used to tip on resonance protons away from the B0 field. • Disturbs or flip all the protons, at the same time, out of alignment from the scanners magnetic field. • The frequency of the RF pulse must be the same as precessional frequency of hydrogen protons, so they can exchange energy, so that they are on resonance • Resonance enables the protons to absorb enough energy from the RF pulse to rotate their axes away from the B0 field, so that the MRI scanner can measure it. Ø B 1 FIELD INCREASES FROM TOE TO HEAD. HYDROGEN NUCLEI IN HEAD SPIN FASTER DUE TO GREATER MAGNETIC FIELD. GREY MATTER, WHITE MATTER, CSF PROTONS SPIN AT VARYING SPEEDS WHEN RF PULSE IS APPLIED. A COIL IS PLACED AROUND THE HEAD TO MEASURE EMITTED ENERGY WHEN RF PULSE IS STOPPED. IMAGE ACQUISITION • When the RF pulse is turned off, the protons flip back and realign along the main magnetic field, BO. As the protons flip back and realign with B0, they give off energy. • Different tissues in the body give off different amounts of energy. A coil is placed around the body part being imaged to detect energy. • The coil acts as an antenna and detects the released energy as an electrical current. The electrical current is transformed, via a computer, using a mathematical calculation called a Fourier transformation (A mathematical calculation that is used to change the electrical current in a coil into an image.) IMAGING PROCESS • The MR imaging process can be divided into a few simple steps.[4] 1.The patient is placed in a magnetic field and essentially becomes a magnet. 2.A radio wave is sent in. 3.The radio wave is turned off. 4.The patient emits a signal 5.The signal is received and used for reconstruction of the picture. WHAT HAPPENS NEXT……….. • The RF pulse is applied only as a brief burst. • After the RF pulse (or wave) is switched off, the longitudinal magnetisation (M2) increases (or recovers), while the transverse magnetisation (Mxy) decreases (or decays) as the protons gradually realign along the Bo direction. • The T1 relaxation time is defined as the time required for recovery of 63% of the magnetisation along the longitudinal direction (i.e.B6) after a 9RF pulse. The T1 recovery rate of a hydrogen proton varies in different tissues due to diverse macromolecular environments. • The T1 relaxation time of a tissue reflects the degree of transfer of RF energy from the recovering spinning protons to the surrounding tissue lattice. Hence, the T1 relaxation time is also known as the spin - lattice relaxation time. • Closely coupled tissues such as fat have short relaxation times "loose" tissue such as CSF have long T1 relaxation time. WHAT HAPPENS NEXT……….. • Following the RF pulse, the dipoles oriented in the transverse plane start to decay. Initially processing at the same rate and in the same direction, the individual dipoles then gradually become out of phase with one another (known as spin - spin interactions). • The time taken for (Mxy) to decay to 37% of its original value is the T2 relaxation time. • The T1 relaxation time reflects the internal local field strength of a particular tissue. Solid tissues such as muscle with a fixed molecular structure and strong local magnetic result in rapid dephasing of dipole movements and therefore gives rise to short T2 relaxation times.[1-3,5] • Liquids, whose molecules are mobile, produce weaker local field strength have longer T2 relaxation times. Fat which has a short T1 appears white or bright an T1 weighted the imaging scans. Because most pathologic processes result in an increase in the amount of free or bulk water, T2 weighted images are used to detect disease and performed before T1 sequences. T1 & T2 WEIGHTED IMAGES T1 AND T2 RELAXATION TIMES • End of RF pulse- net increase in Longitudinal Magnetization- (spin lattice relaxation time); different for differing tissues • At the end of RF pulse- net decrease in Transverse Magnetization- Time required for transverse magnetization to become zero- T2 relaxation time(spin-spin relaxation time); T2 relaxation occurs more rapid than T1 relaxation; SAFETY RISKS • MRI’s create up to 120dB • Equivalent to jet engine at take off. • Contraindications: • Pacemakers, Vagus Nerve Stimulators, implantable defibrillators, insulin pumps, deep brain stimulators • Any electronic or magnetized foreign bodies (surgical prosthesis) • Peripheral nerve stimulation (PNS) • Rapid switching on and off of the magnetic field gradients is capable of causing nerve stimulation MRI INTERPRETATION Dr. SONALI VEDRAJ SHARMA MRI IN TMJ IMAGING • Gold standard for assessing disc position and intraarticular degenerative disorders. • T1 weighted image: Excellent ANATOMIC detail • T2 weighted image: Excellent view of Joint effusion and medullary bone edema • Also used for assessing integrity of neural structures ANTERIOR DISC DISPLACEMENT TMJ DYSFUNCTION WITH JOINT EFFUSION SUBMANDIBULAR GLAND SIALOGRAPHY • 1- Primary duct • 2-Secondary duct branches • 3-Tertiary duct branches DUCT OF RIVINUS CROSSING MAIN SUBMANDIBULAR GLAND DUCT VERTICALLY AXIAL T2 WEIGHTED IMAGE AND MR-SIALOGRAPHY SHOWING SIALOLITH IN MAIN SUBMANDIBULAR DUCT MRI IN IMAGING INTRAORAL HARD AND SOFT TISSUES • Uses Wireless, inductively coupled intraoral coils are used intraorally • This technique allows the display of cancellous bone, gingiva surrounding the teeth and alveolar bone, vestibular and lip mucosa, the periodontal apparatus, the dental pulp and Inferior alveolar nerve. • Simultaneous CBCT did not show gingiva and the mucosa; • Inferior alveolar nerve was delineated because of its bony boundaries • Dental pulp was visible because of surrounding dental hard tissues. A- HISTOLOGICAL SECTION B- MR IMAGE WITH IDENTICAL VISIBLE STRUCTURES C- CBCT IMAGE WITH FEWER VISIBLE STRUCTURES LEFT: MR IMAGE RIGHT: CBCT IMAGE REFERENCES • 1) Magnetic Resonance Imaging of Intraoral hard and soft tissues using an intraoral coil and FLASH sequences. Eur. Radiol (2016) 26: 4616- 4623 Eur Radiol. 2016 Dec;26(12):4616-4623. Epub 2016 Feb 24. • 2) Magnetic Resonance Sialography findings of Submandibular Ducts Imaging. Hindawi Publishing Corporation, Biomed Research International(2013). BioMed Research International Volume 2013, Article ID 417052, 6 pages http://dx.doi.org/10.1155/2013/417052 • Diagnosis of Temporomandibular Joint Disorders; Indications of Imaging Exams. Braz J Otorhinolaryngol. 2016; 82(3): 341-352 Braz J Otorhinolaryngol. 2016 May-Jun;82(3):341-52. doi: 10.1016/j.bjorl.2015.06.010. Epub 2016 Jan 8. • 4) Oral Radiology 6th Edition Principles and Interpretation. Authors: Stuart White Michael Pharoah eBook ISBN: 9780323168175 eBook ISBN: 9780323075923 SCIENCE MAKES LIFE BETTER