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Mri for identifying types of fistulae
This document provides an overview of magnetic resonance imaging (MRI) physics and the MRI appearance of normal anal canal anatomy and various grades of anal fistulas. Key points include: - MRI uses strong magnetic fields and radio waves to produce detailed images based on the magnetic properties of hydrogen protons in tissues. - Normal anatomy includes identification of anal sphincter muscles and mesorectal fascia on MRI images. - Anal fistulas are classified using the Parks or St James's University Hospital system, grading complexity from intersphincteric to transsphincteric or supralevator types. - Examples of each grade are shown with drawings and MRI images, demonstrating internal openings, tracts,
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Mri for identifying types of fistulae
- 1. By Prof. YoussriGaweesh Prof of colorectal surgery Alexandria University
- 2. Historical background Thestory of MRI starts in about 1946 when Felix Bloch proposed that the nucleus behaves like a magnet. In the late 60s Raymond Damadian discovered that malignant tissues had different NMR parameters than normal tissues Clinical Magnetic Resonance Imaging (MRI) uses the magnetic properties of hydrogen and its interaction with both a large external magnetic field and radiowaves to produce highly detailed images of the human body.
- 3. One(1) Tesla isequal to 10,000 Gauss. The magnetic field of the earth is approximately 0.5 Gauss. Given that relationship, a 1.0 T magnet has a magnetic field approximately 20,000 times stronger than that of the earth. Hydrogen has a significant magnetic moment and is nearly 100% abundant in the human body. For these reasons, we use only the hydrogen proton in routine clinical imaging, and that is where we will focus our attention from here on.
- 4. Why MRI WithCT scanners one can produce images with a lot more contrast, which helps in detecting lesions in soft tissue The principle advantage of MRI is its excellent contrast resolution. With MRI it is possible to detect minute contrast differences in (soft) tissue, even more so than with CT images
- 5. The hardware TheMAGNET The RF Coils The Computer (Data Processing)
- 8. To understand physicsexplore the characters of the following
- 9.
- 13. What is Spin? Spin is a fundamental property of nature like electrical charge or mass. Spin comes in multiples of 1/2 and can be + or -. Protons, electrons, and neutrons possess spin. Individual unpaired electrons, protons, and neutrons each possesses a spin of ½ or - ½. • Two or more particles with spins having opposite signs can pair up to eliminate the observable manifestations of spin. • In nuclear magnetic resonance, it is unpaired nuclear spins that are of importance
- 26. Nuclear Spin A nucleusconsists of protons and neutrons • When the total number of protons and neutrons (=mass number A) is odd or the total number of protons is odd, a nucleus has an angular momentum (phi) and hence spin – Ex. Hydrogen (1^H) (1 proton), 13^C • The spin of a nucleus generates a magnetic filed, which has a magnetic moment (mu) • The spin causes the nucleus behave like a tiny magnet with a north and south pole
- 27. Nuclear Spin System Collectionof identical nuclei in a given sample of material (also known as spin packet, a voxel in the imaged volume) • In the absence of external magnetic field, the spin orientations of the nuclei are random and cancel each other • When placed in a magnetic field, the microscopic spins tend to align with the external field, producing a net bulk magnetization aligned with the external field
- 28. • The hydrogenproton can be looked at as if it were a tiny bar magnet with a north and a south pole. • Why Hydrogen ??? • WE HAVE A LOT OF IT • IT HAS GOT THE HIGHEST GYROMAGNETIC RATIO 42.6 MHz/T
- 29. When we puta person in a magnet some interesting things happen to the hydrogen protons: 1. They align with the magnetic field 2. They precess or “wobble” out of phase due to the magnetic momentum of the atom. • ω0 = γ Β0
- 30. Precession Spins PRECESS ata single frequency(w0), but incoherently , they are not in phase, so that the sum of x-y components is 0, with net magnetization vector in z direction W0=gamma B_0: Larmor freq.
- 32. How do weget an image? To obtain an image from a patient it is not enough to put him/her into the magnet. We have to do a little bit more than that. The following steps can be divided into: Excitation Relaxation Acquisition Computing and Display.
- 33.
- 34. The field ofthe RF coil B1 is perpendicular to B0 original field
- 35. Relaxation We rotatedthe net magnetization 90o into the X-Y plane. We could also say that we lifted the protons into a higher energy state, same thing. This happened because the protons absorbed energy from the RF pulse. Protons rather be in a low energy state. Now something happens that is referred to as Relaxation. The relaxation process can be divided into two parts: T1 and T2 relaxation.
- 36.
- 37. T2 Relaxation •When we apply the 90o RF pulse something interesting happens. Apart from flipping the magnetization into the X-Y plane, the protons will also start spinning in-phase!!
- 39. Remember this: •T1 andT2 relaxation are two independent processes, which happen simultaneously. •T1 happens along the Z-axis; T2 happens in the X-Y plane. •T2 is much quicker than T1 •Every tissue has its built in T1 and T2 relaxation times. •T2 is much smaller than T1 – For tissue in body, T2: 25-250ms, T1: 250- 2500 ms
- 40. Formation of SpinEcho By applying a 180 degree pulse, the dephased spins can recover their coherence, and form an echo signal
- 41.
- 42. Sequence Parameters: TR= Repetition time TE= Echo time Flip Angle Matrix Field of View Slice thickness
- 43. Spin echo sequence T_R (pulse repetition time) • Multiple π pulses create “Carr-Purcell-Meiboom-Gill (CPMG)” sequence • Echo Magnitude Decays with time constant T2
- 44.
- 45. Basic Principle ofMRI The hydrogen (1^H) atom inside body possess “spin” • In the absence of external magnetic field, the spin directions of all atoms are random and cancel each other. • When placed in an external magnetic field, the spins align with the external field. • By applying an rotating magnetic field in the direction orthogonal to the static field, the spins can be pulled away from the z-axis with an angle alpha • The bulk magnetization vector rotates around z at the Larmor frequency (precess) • The precession relaxes gradually, with the xy-component reduces in time, z-component increases • The xy component of the magnetization vector produces a voltage signal, which is the NMR signal we measure
- 46. Process Involved inMRI Put patient in a static field B_0 (much stronger than the earth’s field) • (step 1) Wait until the nuclear magnitization reaches an equilibrium (align with B_0) • Applying a rotating magnetic field B_1 (much weaker than B_0) to bring M to an initial angle alpha with B_0 (rotating freq=Larmor freq.) • M(t) precess around B_0 at Larmor frequency around B_0 axis (z direction) with angle alpha • The component in z increases in time (longitudinal relaxation) with time constant T1 • The component in x-y plane reduces in time (transverse relaxation) with time constant T2 • Measure the transverse component at a certain time after the excitation (NMR signal) • Go back to step 1 • By using different excitation pulse sequences, the signal amplitude can reflect mainly the proton density, T1 or T2 at a given voxel
- 47. Image Weighting Hydrogen infat recovers faster than that in water in the Z axis and loses phase faster in the X-Y axis. T1 & T2 time in fat is shorter than water T2 time of fat is 80ms and water is 200ms T1 Contrast and T2 contrast Long TR and TE….T2W image Short TR and TE….T1W image Long TR and Short TE….PD image
- 48. T1 & T2weighting •Fat and Water are hyperintense on T2 images •Most pathological processes have T1 hypointense and T2 hyperintense (altered fluid contents). •Air, cortical bone, dense fibrous structures are hypointense on T1 & T2 images. •T1 hyperintense signal in: •Fat •Calcium (sometimes) •Melanin •Subacute blood (metHb) •High protein fluid •Flowing fluid
- 49. T1 weighted image Fatis bright Water/simple fluid is dark Cerebral gray matter is grey Cerebral white matter is white Other materials are also bright : acute hemorrhage (1-3 days old) , melanine , hydrated calcium, proteinaceous material and gadolinium
- 50. T2 weighted image Fatis bright (less than that in T1) Water/simple fluid is bright Cerebral grey mater is grey Cerebral white matter is dark
- 51. Normal anatomy a AxialT2-weighted of the pelvis depicting the layers of the rectal wall. The mucosa and submucosa can be visualized as a relatively hyperintense band (arrows). The hypointense line (arrowheads) represents the muscularis propria. b Axial T2- weighted sequence. The mesorectal fascia can be visualized as a thin line (arrowheads), enveloping the mesorectal compartment, containing the rectum, mesorectal fat, bloodvessels, lymphatic vessels and nodes
- 52. A coronal diagramdepicting the two anatomical levels (1 and 2) of the distal rectum to help define the surgical approach
- 53. Coronal T2-weighted MR imageshows the normal anatomy of the rectum. The white line indicates the lower limit of the rectum at the insertion of the levator ani muscle (arrows) on the rectal wall. The levator ani muscle forms the ceiling of the ischiorectal fossa.
- 54. Normal anatomy ofthe mesorectum. (a) Axial T2- weighted MR image shows the mesorectal fascia as a thin, hypointense layer (white arrowheads) surrounding hyperintense mesorectal fat. On the anterior aspect, the mesorectal fascia appears more thickened and is difficult to differentiate from the Denonvillier fascia (black arrowheads (b) Photograph of a section of the explanted rectum shows perirectal fat surrounded by the mesorectal fascia.
- 55. Coronal T2-weighted MRimage obtained with a phased-array surface coil shows a normal anal sphincter complex. The levator ani muscle (straight arrows) appears as a funnel- shaped muscular layer that extends from the obturator ani muscle to the anal canal. The puborectalis muscle (arrowheads) is depicted at the insertion of the levator ani muscle onto the anal canal. The external (curved arrows) and internal (*) sphincter muscles are also seen.
- 56. On MRI themesorectal fat has a high signal intensity on T1- and T2-weighted images. The mesorectal fat is bounded by the mesorectal fascia, which is seen as a fine line of low signal intensity (red arrows). .
- 57. Normal male anatomy.Drawing (a)
- 58. T2-weighted MR image(b) show the normal male anatomy of the perineum at the level of the mid anal canal (AC in b) in the axial plane. In b, ES = external sphincter, IA = ischioanal fossa, InS = intersphincteric space, IS = internal sphincter.
- 59. Normal female anatomy.Drawing (a)
- 60. T2-weighted MR image(b) show the normal female anatomy of the perineum at the level of the proximal half of the anal canal (AC in b) in the axial plane. In b, ES = external sphincter, InS = intersphincteric space, IO = internal obturator muscle, IR = ischiorectal fossa, IS = internal sphincter, U = urethra, V = vagina.
- 61. Drawing shows thenormal anatomy of the anal canal in the coronal plane.
- 62. Suggested orientation foraxial MR imaging of the anal canal. Sagittal T2- weighted image through the midline is used to obtain images that are truly axial relative to the anal canal
- 63. Suggested orientation forcoronal MR imaging of the anal canal. Coronal MR imaging is performed at 90° relative to the axial plane to obtain images parallel to the long axis of the anal canal.
- 64. Anal clock. AxialT2-weighted MR image of the male perineum shows the anal clock diagram used to correctly locate anal fistulas with respect to the anal canal. AP = anterior perineum, L = left aspect of the anal canal, NC = natal cleft, R = right aspect of the anal canal.
- 65. Parks classification. Drawingof the anal canal in the coronal plane shows the Parks classification of perianal fistulas. A = intersphincteric, B = transsphincteric, C = suprasphincteric, D = extrasphincteric. The external sphincter is the keystone of the Parks classification.
- 66. St James’s UniversityHospital Classification The classification grades fistulas into five groups: grade 1, simple linear intersphincteric fistula; grade 2, intersphincteric with abscess or secondary track; grade 3, transsphincteric; grade 4, transsphincteric with abscess or secondary track in ischiorectal or ischioanal fossa; grade 5, supralevator and translevator
- 67. Grade 1: simplelinear intersphincteric fistula. (a) Drawing of the anal canal in the axial plane shows a simple intersphincteric fistula at the 2-o’clock position (arrow).
- 68. (b) Axial contrast-enhancedfat-suppressed T1- weighted MR image shows the left intersphincteric fistula (arrow) bounded by the external sphincter without a secondary fistulous track or abscess.
- 69. Grade 1: simplelinear intersphincteric fistula (same patient as in pevious Fig). (a) Drawing of the anal canal in the coronal plane shows the simple intersphincteric fistula to the left of the anal canal.
- 70. (b) Coronal contrast-enhancedfat-suppressed T1-weighted MR image shows the highly enhancing intersphincteric fistula (arrow) confined by the external sphincter.
- 71. Grade 2: intersphinctericfistula with an abscess. (a) Axial drawing of the anal canal shows a right posterolateral abscess (arrow).
- 72. (b) Axial T2-weightedMR image shows the high-signal-intensity fluid collection along the right posterolateral aspect of the anal canal (arrow).
- 73. (c) Axial contrast-enhancedfat-suppressed T1-weighted MR image shows the abscess in the right posterolateral aspect of the intersphincteric space (arrowhead), bounded by the external sphincter.
- 74. Grade 2: intersphinctericfistula with an abscess (same patient as in previous Fig ). (a) Coronal drawing of the anal canal shows the abscess in the intersphincteric space (arrow), bounded by the external sphincter.
- 75. (b) Coronal contrast-enhancedfat-suppressed T1-weighted MR image shows the right intersphincteric abscess (arrow) without a fistulous track or abscess in the right ischiorectal fossa.
- 76. Grade 3: transsphinctericfistula. (a) Axial drawing of the anal canal shows a posterior transsphincteric fistula (arrow) with the internal opening at the 6-o’clock position. (
- 77. (b) Axial contrast-enhancedfat-suppressed T1- weighted MR image shows the transsphincteric fistula (arrow) crossing the external sphincter.
- 78. Grade 3: transsphinctericfistula (same patient as in previous Fig ). (a) Coronal drawing of the anal canal shows the right transsphincteric fistula.
- 79. (b) Coronal contrast-enhancedfat-suppressed T1-weighted MR image shows the highly enhancing transsphincteric fistula (arrow) from the dentate line to the skin, passing through the ischioanal fossa and piercing the external sphincter.
- 80. Grade 4: transsphinctericfistula with an abscess or secondary track in the ischiorectal or ischioanal fossa. (a) Axial drawing of the anal canal shows a posterior transsphincteric fistula with an abscess in the right ischiorectal fossa.
- 81. (b) Axial T2-weightedMR image shows the transsphincteric fistula crossing the external sphincter at the 6-o’clock position (arrow) and a high-signal-intensity fluid collection in the right ischiorectal fossa (arrowheads).
- 82. (c) Axial contrast-enhancedfat-suppressed T1- weighted MR image shows the posterior transsphincteric fistula (straight arrow), the abscess in the right ischiorectal fossa with nonenhancing pus in the cavity (arrowheads), and a secondary extension in the left ischiorectal fossa (curved arrow).
- 83. Grade 4: transsphinctericfistula with an abscess or secondary track in the ischiorectal or ischioanal fossa (same patient as in previous Fig ). (a)
- 84. (b) Coronal contrast-enhancedfat-suppressed T1-weighted MR image shows the abscess in the right ischiorectal fossa with nonenhancing pus in the cavity (arrowheads) and the secondary extension in the left ischiorectal fossa (arrow).
- 85. Grade 5: supralevatorand translevator disease. (a) Axial drawing of the anal canal shows a supralevator abscess located at the urethra (U), the left side of the anal canal, and the left internal obturator muscle (IO).
- 86. (b) Axial contrast-enhancedfat-suppressed T1-weighted MR image shows the left supralevator abscess with inflammatory changes in the left internal obturator muscle (arrows).
- 87. Grade 5: supralevatorand translevator disease (same patient as in previous Fig ). (a) Coronal drawing of the anal canal shows the left supralevator abscess with a left translevator fistula.
- 88. (b) Coronal contrast-enhancedfat-suppressed T1-weighted MR image shows the left supralevator abscess with inflammatory changes surrounding the rectum and the left translevator fistula crossing the ischiorectal fossa (arrowheads).
- 89. Horseshoe abscess. AxialT2-weighted MR image shows a horseshoe abscess with a fluid-fluid level in both ischiorectal fossae (arrowheads). The abscess has high signal intensity due to pus and a liquid-liquid level due to detritus.
- 90. Horseshoe abscess (samepatient as in previosu Fig ). Axial (a) and coronal (b) contrast-enhanced fat-suppressed T1-weighted MR images show a horseshoe abscess in the ischiorectal and ischioanal fossae (arrows in a, arrowheads in b). The abscess has intense enhancement due to the presence of active inflammatory tissue.