Introduction to mri

Introduction to MRI
Basic hardware and sequences.

1. Introduction to MRI Dr. Rajesh Venunath Nair Assistant Professor Department of Radiodiagnosis K.S Hegde Medical Academy, Mangalore

2. Objectives To know the history behind the discovery of magnetic resonance imaging. To understand the principles of MR imaging and the steps and basic physics behind image acquisition. To understand the basic hardware and soft ware involved in MR imaging. To elaborate the advantages, disadvantages and uses of MRI. To understand the safety issues associated with MRI. To familiarize with the latest application of MRI.

3. Introduction to MRI M agnetic R esonance I maging

4. Types of MR Scanners Open MRI – Low Magnetic field strength made of permanent magnets Closed MRI – Superconducting magnets of high field strength.

5. Parts of MRI Scanner

6. Types of RF Coils Volume coils – Head coil, Body coil, Spine coil. Surface coils Phase array coils. Surface coils produce high resolution images but the field of view is small. Body coils have larger field of view. RF coils can be receiving only, transmitting only or combined receiving and transmitting coils. RF pulses commonly used are 90 and 180 degree pulses.

7. Steps in MRI Image Acquisition

8. Understanding and Reading MRIs

9. Numbers

10. Algorithim Fournier’s Transformation. Conversion of radiofrequency signals into images

11. Reading the Images

12. The emitted energy of the protons once the RF is stopped is affected by in which tissue (the “ lattice ”) it resides: fat, muscle, ligament, bone, brain, etc.

13. RF Pulse sequence parameters which the technician adjusts The three pulse sequence parameters are Repetition time ( TR ) measured in msec ie the time to repeat the RF pulse. Echo time ( TE ) measured in msec ie the time to receive the signal. Time to invert( TI ) ie the time to sent the 180degree inversion pulse. Flip angle measured in degrees ie the degree to which the spinning protons are flipped from the horizontal axis.

14. T1 and T2 relaxation times Occur simultaneously and independently of each other and form the basis of tissue contrast in MR-reconstructed images

15. T1 Low TR (400-700 msec ) Low TE (20-40 msec ) T2 High TR (2,000-3,000 msec ) High TE (40-70 msec ) Proton density High TR (2,000-3,000msec) Low TE (20-40 msec )

16. Basic Sequences T1 to view anatomy T2 to detect a pathologic process (edema, swelling) Proton Density (PD) great for ligamentous anatomy

17. Basic Pulse Sequences for MRI Imaging Image type Fat Water Advantage T1 Bright Dark Anatomical detail T2 Intermediate Bright ++ edema Fat Suppressed T2 Very Dark Very Bright ++++ edema

18. Advantages of MRI Lack of ionizing radiation – used in pediatric and obstetric imaging. Better soft tissue contrast resolution. Multiplanar imaging capabilities. Does not use iodinated contrast agents. Safe in patients with renal failure.

19. Disadvantages of MRI Expensive. Longer scanning time. Susceptible to movement artifacts. Cannot be used in acute trauma settings. Does not image the bone and lungs. Scanner not available at all places. Requires skilled technicians for operating and maintenance cost is high.

20. Indications for MR Imaging Diagnosis of Brain and Spinal cord diseases. Staging of cancers like rectum, cervix and prostate. Diagnosis of musculoskeletal pathologies of knee, hip and shoulder. Imaging of spine and intervertebral discs. Fetal MRI to detect anomalies. Cardiac MRI for assessment of myocardium. Breast MRI for diagnosing breast lesions.

21. Contraindications for MRI Absolute: Patients with cardiac pacemaker or defibrillator. Patients with intraoccular metallic foreign bodies. Patients with cochlear implants. Patients with stainless steel metallic implants. Patients with souvenir bullets. Relative: Claustrophobic patients. Patients with Insulin Pump. Patients with knee and hip prosthesis.

22. Advantages of MRI over CT

23. MRI Contrast Agent Gadolinium chelates are the commonly used MRI contrast agenta and the next being Manganese. They are paramagnetic elements with atomic number 64 with 7 unpaired electrons. It reduces the T1 relaxation time of protons and increases the signal intensity on T1WI. Given as intravenous agents at dose of 0.1mg/k, they are intravascular agents that improves the conspicuity of lesions which further aids in confirming the diagnosis. Usually viewed in T1 weighted fat suppressed images in multiplanar sections. Side Effects – Nausea Vomiting, Anaphylaxis and Seizures.

24. Advantages of Fat Suppression

25. Uses of Gadolinium MR Arthrography . Enhancement of tumor masses and inflammatory abscess. MR Urography . Contrast Enhanced MR Angiography. Dreaded complication of long term gadolinium use is nephrogenic systemic fibrosis in patients with renal failure and reduced GFR.

26. MRI Safety

27. Recent Advances in MRI Diffusion weighted images based on the restricted movement of water protons, useful in the diagnosis of stroke and tumors of brain. MR Spectroscopy – Non invasive method to assess the metabolites in tissues based on the spin of protons within each metabolite. Functional MRI – gives information about the functional activity of different parts of brain using BLOD sequence. MRI guided biopsy.

28. Visual cortex activation in functional MRI.