MRI for Surgeons introduction and basics

Introduction to MRI for surgeons

1. MRI for Surgeons Rohit Sharma MD 12/7/2023

2. Contents History of NMR Basics of MRI Signal Localization Image Contrast MRI Brain MRCP MRI for Liver SOL MRI Breast MR angiography MRI Fistulogram /Defecography MRI Soft tissue Sarcoma Take Home Points

3. Professor Isidor I Rabi ( 1898-1988) 1937 : Discovered quantum phenomenon of Nuclear Magnetic Resonance (NMR) Atomic nuclei absorb and emit radio waves when exposed to strong external magnetic field 1944 Physics Nobel Worked on the Manhattan Project

4. Raymond Damadian Physician discovered hydrogen signal in cancer is different from that of healthy tissue

5. On the 2003 Nobel Prize in medicine or physiology awarded to Paul C. Lauterbur and Sir Peter Mansfield Magnetic Resonance in Med, Volume: 51, Issue: 1, Pages: 1-3, First published: 29 December 2003, DOI: (10.1002/mrm.10717)

6. Basic Of MRI Magnetic Resonance Imaging (MRI) produces images of body's internal structures by passing radio waves through powerful magnetic field Differing frequencies of radio waves are produced by the different body structures, and are mapped converted into digital images by a computer MRI is very suitable for imaging soft tissue MRI has advanced beyond a tomographic imaging technique to a volume imaging technique

7. Elemental 0f 102 naturally occurring elements, only 26 are found in human body 96% formed by Oxygen, Hydrogen (H 2 O), Carbon, Nitrogen Consider only Hydrogen in rest of the talk Simplest element with Atomic Number1 and Atomic Weight 1 ( 1 H 1 ) Ionic Hydrogen is a solitary proton without electron (H + ) Positively charged proton also has a magnetic spin (Wobble) MRI utilizes the magnetic spin property of Hydrogen Ions to elicit images

8. Why Don’t We Act Like Magnets? The Hydrogen Ions are spinning in hap hazard manner and different orientation cancel out a directional magnetism vector They need discipline !

9. Need a Strong Magnet Externally 1 Tesla (T) = 1000 Gauss Earths magnetic field =0.5 Gauss Clinical Utility Magnets =0.3-7 T 2500 times More than Earths magnetic Field Average Field Strength = 1.5T Open Magnet = Less field strength, less claustrophobic Closed Magnet = More field Strength, Claustrophobic

10. Proton Alignment

11. Precession Each hydrogen nucleus is spinning on its axis The influence of Bo ( external magnetic field) produces an additional spin or wobble of the magnetic moments of hydrogen around Bo This secondary spin is Precession and causes the magnetic moments to follow a circular path around Bo

12. Resonance When an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation The nucleus will gain energy from external force Nucleus resonates if the (RF pulse) energy is delivered at the same precessional frequency (LARMOR FREQUENCY) If the energy is delivered at different frequency to that of LARMOR frequency of the nucleus , resonance does not occur Same principle applies to Opera singers breaking glass with their Voice!

13. What is a Radio Frequency Pulse? Same as Radio Waves- high wavelength, low energy electromagnetic waves Radiofrequency coils Act as a transmitter and receiver Different types of coils

14. Signal Creation, Detection, Transmission and Image Generation Short burst of radio waves are sent from the scanner to knock the protons from their position The protons realign when RF pulse stop The protons realign at different speeds in different tissues Hence signal emitted varies from different tissues Detector on the scanner receive the signals Transmission to computer Computer creates picture based on radio signals

15. Schematic representation of MRI

16. Basic Imaging Sequences Magnetization Gradient allow the direction of signal to be known Allow Each point in space to be distinguishable in the XYZ spatial encoding Slice selection (Z), Phase encoding (X), Frequency Encoding (Y) K Space where data is stored T1 weighting ( “spin lattice” relaxation) relies on longitudinal relaxation of net magnetization vector (NMV) T2 weighting ( “spin-spin” or transverse relaxation) of NMV

17. Pulse Sequence In CT the contrast depends entirely on electron density In MRI contrast is a complex function of Proton density, T1 relaxation, T2 relaxation and local chemical environment Manner of tissue excitement influences the image The temporal pattern and shape of RF, Gradient coil waveforms used to obtain image is referred to as Pulse Sequence Spin Echo ( “ black blood”) imaging- suppress imaging from flowing blood while maintaining high signal in the surrounding stationary tissues- used to study the parenchyma Gradient Echo ( “ bright blood”) imaging- MRA, highlights the flowing blood

18. MR Contrast Agents Paramagnetic ( weakly attracted by external magnetic field and form internal, induced magnetic field in direction of applied external) Gadolinium derivatives IV administration Hyperpolarized gases like He 3 and Xe 129 can be used in detection of ventilation abnormalities ( Cystic Fibrosis) Renal excretion Anaphylactoid reaction are rare. Nephrogenic Systemic Fibrosis (NSF)

19. Contrast Enhanced MRI Contrast media Gadolinium (most common) Iron oxide Magnafodipir trisodium (hepatobiliary specific) Indications CNS neoplasm CNS infection Differentiate between scar and disc in spine In neoplasms of other parts of body T1-w large ill-defined SOL Postcontrast peripheral enhancement of the tumor

20. Terminologies High signal intensity/ hyperintense = White Intermediate signal intensity/ isointense = Grey Low signal intensity/ hypointense = Black

21. Terminologies T1 T2

22. BLACK DARK GREY intermediate) WHITE Air, calcium, cortical bone, rapidly flowing blood Fluid, ligaments/tendon, abdominal organs, cartilage High protein tissue (abscess, cyst) Grey matter Fat, blood, gadolinium (contrast) T1W Imaging

23. BLACK DARK GREY WHITE Air, calcium, cortical bone, rapidly flowing blood Ligaments/tendon, abdominal organs, cartilage Fat, cartilage Free fluid, water T2W Imaging

24. Which scan to do T1 W Images: (best for anatomical details) Subacute Hemorrhage , Fat-containing structures T2 W Images : (best for pathological details) Edema , Demyelination, Infarction, Chronic Hemorrhage FLAIR ( Fluid Attenuated Inversion Recovery) Images: Edema , Demyelination, Infarction esp. in Periventricular location

25. Indications of FLAIR 1. Extent of perilesional edema can be determined easily Brain infarctions are well seen on FLAIR Fast FLAIR shows subarachnoid hemorrhage Syrinx/cysts in spinal cord are well seen on FLAIR. T2W FLAIR

26. Diffusion Weighted Imaging INDICATIONS Earliest diagnosis of stroke Differentiating epidermoid (bright) from arachanoid cyst (intensity of CSF) Differentiating abscess (bright) from simple cyst (same as CSF) CNS neoplasms- helps in prognostication Ca breast- differentiation of DCIS from invasive ductal carcinoma BLEED

27. Flow Void (Signal Void) Due to fast moving hydrogen in blood, it is unable to align itself with external magnetic field giving rise to a dark signal- flow void When contrast is given the ferromagnetic substances (Gadolinium) facilitate this alignment Plain Contrast Right Jugular Block

28. SDH T1W T2W FLAIR

29. Saggital Sinus Thrombosis MR-Venogram- absence of flow in right transverse sinus Sigmoid sinus Superior saggital sinus Transverse sinus

30. SOL brain T2W FLAIR T1W T1W CONTRAST

31. Spinal cord CSF disc Vertebral body Spinal cord CSF disc Vertebral body MRI- Normal Spine

32. Spine pathologies on MRI Lumbar disc herniation (T2W) Kyphoscoliosis (T2W)

33. Spine Pathologies Intramedullary Tumor (T2W) Tethered Cord Syndrome (T1W)

34. MRI for Liver SOL SPIO ( superparamagnetic iron oxide) is the imaging agent for Liver lesions Different agents for different indications : Discuss with Radiologist prior to ordering

35. MRCP Indications: Pathology of common bile duct, pancreatic duct and pancreas

36. Choledocholithiasis Chronic Pancreatitis Filling defect in CBD Dilated pancreatic duct Chain of lake appearance

37. D= duodenum MPD= main pancreatic duct APD= annular pancreatic duct Annular Pancreas Ca Ampulla PSC Double duct sign Red- CBD Yellow-MPD Blue- pancreatic mass String of beads appearance

38. Choledochal Cyst Type 1 Type 2 Type 3 Type 4 Type 5

39. Current Indication of Breast MRI High risk breast cancer in females adjunctive screening Detection of occult primary in patient with axillary metastases Assessment of breast implants ( Silicone, Free Silicone Injections, Saline) Pre-operative evaluation in newly diagnosed breast cancer Measurement of response to neoadjuvant Chemotherapy Differentiation postoperative changes from recurrence Evaluation of inconclusive clinical or imaging findings Scarred breast, residual tumor, Pec Major invasion in large or posterior tumors

40. MRI in CA Breast Indications Distinguish scar from recurrence in women who have had previous breast conserving surgery To assess multifocality and multicentricity in lobular Ca breast To assess extent of high grade DCIS Best imaging modality in females with implants Screening tool in high risk women

41. Contra Indications Cardiac pacemakers, cochlear implants/ other metals in body Unable to lie prone Large body habitus who can not fit into bore of magnet Large breast can not be accommodated into breast coils Extreme claustrophobia

42. Limitation of Breast MRI Water content of breast influence by menstrual cycle Both diffuse and focal enhancement can occur in fourth and first week of menstrual cycle especially in young patient Ideal between second and third week (6-16 days) of cycle Lactation Cancer treatment Menopausal status Hormone replacement therapy

43. Interpretation of Breast MRI FOCUS( FOCI): area of enhancement<5mm too small to characterize MASS: 3D SOL, shape, margin, internal characteristics, enhancement pattern NON MASS LIKE ENHANCEMENT: without detectable 3D mass, distribution, internal enhancement pattern, symmetry

44. T2 Fat Suppressed High signal on T2 fat sat- looks for water Cysts, lymph nodes and fat necrosis are bright Colloid Carcinoma is also bright- exception to rule that all things with T2 bright on fat suppressed images are benign Multiple Breast Cysts Fibroadenoma Colloid Carcinoma

45. Ca Breast Multifocal disease in Ca breast Extensive disease in ca breast (Invasive Ductal)

46. Mass Enhancement Pattern HOMOGENOUS: Uniform and confluent e.g. IDC HETEROGENOUS: Non uniform varies within mass e.g. ILC RIM : concentrated on mass periphery ( High grade IDC, fat necrosis, Inflammatory cyst) 40% chance of malignancy DARK INTERNAL SEPTATIONS: non enhancing, typical of fibroadenomas/ phylloides ENHANCING INTERNAL SEPTATION: Malignancy CENTRAL : pronounced enhancement within an enhancing mass, high grade cancer

47. Non Mass Enhancement Can occur in significant number of cancer DCIS Stromal Fibrosis

48. Scar vs Recurrence Recurrence of ca breast Recurrence of ca br – fat saturation

49. Coronary MR Angiography RCA stenosis LAD stenosis PVD Left common iliac artery occlusion

50. MRA Non invasive No Contrast Required Non Ionizing Conventional Angio Invasive Iodinated contrast required- Nephrotoxic Ionizing radiation

51. MRA Indications Carotid : symptomatic/ Asymptomatic for CEA, Aneurysm, Arterial Dissection Intracranial Aneurysms AVM : known follow up, suspected rupture Vertebrobasilar syndrome

52. MRA Principles Time of Flight (TOF) MRA: is due to difference in magnitude of inflowing protons in blood and the surrounding stationary protons No contrast agent needed Motion artifact Difficult with slow flow Phase Contrast (PC) MRA: Directional flow encoding magnetic field gradient proportional to velocity in bipolar gradient Allows quantification of blood flow and velocity More Time consuming Contrast Enhanced (CE) MRA: With gadolinium contrast Can be done in seconds

53. MR- Venography CE MRI T1- low intensity signal ? thrombus MR- venogram- patent transverse sinus

54. MR Fistulogram Intersphinteric fistula Trans-sphincteric fistula Supra-sphincteric fistula Internal and external sphincters are not separately resolved Sphincter complex, ischio-anal fossae, levator sling are clearly seen

55. Soft Tissue Sarcoma Accurate anatomical location Neurovascular invasion Helps in staging, therefore helps provide a roadmap for management T2W image showing liposarcoma right upper limb T1W image showing liposarcoma right shoulder

56. MRI in Congenital Malformations Chiari Malformation Spina Bifida Mullerian Duplication Renal Duplication Anorectal Malformation- Sphincter and rectal location

57. Take home message Basic knowledge about reading MRI is essential in surgical training Identifying the sequence is the first step in learning to read MRI In neoplastic tissue, it helps to determine the vascularity, staging Useful in differentiating scar and recurrence in CA breast and is the best modality in females with breast implants DWI helps in identifying stroke within minutes of an episode Despite all of these benefits a major drawback is the amount of time taken to generate an image