By Prof. Youssri GaweeshProf of colorectal surgery Alexandria University
Historical background The story 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.
One(1) Tesla is equal to 10,000 Gauss. The magnetic field ofthe earth is approximately 0.5 Gauss. Given thatrelationship, a 1.0 T magnet has a magnetic fieldapproximately 20,000 times stronger than that of theearth.Hydrogen has a significant magnetic moment and is nearly100% abundant in the human body. For these reasons, we useonly the hydrogen proton in routine clinical imaging, and thatis where we will focus our attention from here on.
Why MRI With CT 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
The hardware The MAGNET The RF Coils The Computer (Data Processing)
To understand physics explore the characters of the following
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
Nuclear SpinA nucleus consists of protons and neutrons• When the total number of protons andneutrons (=mass number A) is odd or thetotal number of protons is odd, a nucleushas an angular momentum (phi) andhence spin– Ex. Hydrogen (1^H) (1 proton), 13^C• The spin of a nucleus generates amagnetic filed, which has a magneticmoment (mu)• The spin causes the nucleusbehave like a tiny magnet with anorth and south pole
Nuclear Spin SystemCollection of identical nuclei in a given sample ofmaterial (also known as spin packet, a voxel in theimaged volume)• In the absence of external magnetic field, the spinorientations of the nuclei are random and cancel eachother• When placed in a magnetic field, the microscopic spinstend to align with the external field, producing a net bulkmagnetization aligned with the external field
• The hydrogen proton 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
When we put a person in a magnet some interesting things happen to the hydrogen protons:1. They align with the magnetic field2. They precess or “wobble” out of phase due to the magnetic momentum of the atom.• ω0 = γ Β0
PrecessionSpins PRECESS at a singlefrequency(w0), but incoherently, they are not in phase, sothat the sum of x-y components is0, with net magnetization vectorin z directionW0=gamma B_0: Larmor freq.
How do we get an image?To obtain an image from a patient it is notenough to put him/her into the magnet. Wehave to do a little bit more than that.The following steps can be divided into: Excitation Relaxation Acquisition Computing and Display.
The field of the RF coil B1 is perpendicular to B0original field
Relaxation We rotated the 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.
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!!
Remember this:•T1 and T2 relaxation are two independentprocesses, which happen simultaneously.•T1 happens along the Z-axis; T2 happens inthe X-Y plane.•T2 is much quicker than T1•Every tissue has its built in T1 and T2relaxation times.•T2 is much smaller than T1 – For tissue in body, T2: 25-250ms, T1: 250- 2500 ms
Formation of Spin EchoBy applying a 180 degree pulse, the dephased spins can recover theircoherence, and form an echo signal
Basic Principle of MRIThe hydrogen (1^H) atom inside body possess “spin”• In the absence of external magnetic field, the spin directions of allatoms are random and cancel each other.• When placed in an external magnetic field, the spins align with theexternal field.• By applying an rotating magnetic field in the direction orthogonal tothe static field, the spins can be pulled away from the z-axis with anangle alpha• The bulk magnetization vector rotates around z at the Larmorfrequency (precess)• The precession relaxes gradually, with the xy-component reduces intime, z-component increases• The xy component of the magnetization vector produces a voltagesignal, which is the NMR signal we measure
Process Involved in MRI 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
Image WeightingHydrogen in fat recovers faster than that inwater in the Z axis and loses phase faster in theX-Y axis.T1 & T2 time in fat is shorter than waterT2 time of fat is 80ms and water is 200msT1 Contrast and T2 contrast Long TR and TE….T2W image Short TR and TE….T1W image Long TR and Short TE….PD image
T1 & T2 weighting•Fat and Water are hyperintense on T2 images•Most pathological processes have T1 hypointenseand T2 hyperintense (altered fluid contents).•Air, cortical bone, dense fibrous structures arehypointense on T1 & T2 images.•T1 hyperintense signal in: •Fat •Calcium (sometimes) •Melanin •Subacute blood (metHb) •High protein fluid •Flowing fluid
T1 weighted imageFat is brightWater/simple fluid is darkCerebral gray matter is greyCerebral white matter is whiteOther materials are also bright : acutehemorrhage (1-3 days old) , melanine , hydratedcalcium, proteinaceous material and gadolinium
T2 weighted imageFat is bright (less than that in T1)Water/simple fluid is brightCerebral grey mater is greyCerebral white matter is dark
Normal anatomya Axial T2-weighted of the pelvis depicting the layers of the rectal wall. The mucosaand submucosa can be visualized as a relatively hyperintense band (arrows). Thehypointense 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 therectum, mesorectal fat, bloodvessels, lymphatic vessels and nodes
A coronal diagram depicting the two anatomical levels (1 and 2) of the distalrectum to help define the surgical approach
Coronal T2-weighted MRimage shows the normal anatomy ofthe rectum. The white line indicatesthe lower limit of the rectum at theinsertion of the levator ani muscle(arrows) on the rectal wall. Thelevator ani muscle forms the ceilingof the ischiorectal fossa.
Normal anatomy of the mesorectum.(a) Axial T2- weighted MR imageshows the mesorectal fascia as athin, hypointense layer (whitearrowheads)surrounding hyperintensemesorectal fat. On the anterioraspect, the mesorectal fascia appearsmore thickened and is difficult todifferentiate from the Denonvillierfascia (black arrowheads (b)Photograph of a section of theexplanted rectum showsperirectal fat surrounded by themesorectal fascia.
Coronal T2-weighted MR imageobtained with a phased-array surfacecoil shows a normal anal sphinctercomplex. The levator ani muscle(straight arrows) appears as a funnel-shaped muscular layer that extendsfrom the obturator ani muscle to theanal canal. The puborectalis muscle(arrowheads) is depicted at theinsertion of the levator ani muscle ontothe anal canal. The external (curvedarrows) and internal (*) sphinctermuscles are also seen.
On MRI the mesorectal fat has a high signal intensity on T1- and T2-weightedimages.The mesorectal fat is bounded by the mesorectal fascia, which is seen as a fine lineof low signal intensity (red arrows)..
T2-weighted MR image (b) show the normal male anatomy of the perineumat 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.
T2-weighted MR image (b) show the normal female anatomy of the perineumat 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 = internalobturator muscle, IR = ischiorectal fossa, IS = internal sphincter, U =urethra, V = vagina.
Drawing shows the normal anatomy of theanal canal in the coronal plane.
Suggested orientation for axial MR imaging of the anal canal. Sagittal T2-weighted image through the midline is used to obtain images that are trulyaxial relative to the anal canal
Suggested orientation for coronal MR imaging of the anal canal. Coronal MRimaging is performed at 90° relative to the axial plane to obtain imagesparallel to the long axis of the anal canal.
Anal clock. Axial T2-weighted MR image of the male perineum shows theanal clock diagram used to correctly locate anal fistulas with respect to theanal canal. AP = anterior perineum, L = left aspect of the anal canal, NC =natal cleft, R = right aspect of the anal canal.
Parks classification. Drawing of the anal canal in the coronal plane showsthe Parks classification of perianal fistulas. A = intersphincteric, B =transsphincteric, C = suprasphincteric, D = extrasphincteric. The externalsphincter is the keystone of the Parks classification.
St James’s University Hospital ClassificationThe classification grades fistulas into fivegroups: grade 1, simple linearintersphincteric fistula; grade2, intersphincteric with abscess orsecondary track; grade 3, transsphincteric;grade 4, transsphincteric with abscess orsecondary track in ischiorectal orischioanal fossa; grade 5, supralevator andtranslevator
Grade 1: simple linear intersphincteric fistula. (a) Drawing of the analcanal in the axial plane shows a simple intersphincteric fistula at the2-o’clock position (arrow).
(b) Axial contrast-enhanced fat-suppressed T1- weighted MR image showsthe left intersphincteric fistula (arrow) bounded by the external sphincterwithout a secondary fistulous track or abscess.
Grade 1: simple linear intersphincteric fistula (same patient as in peviousFig). (a) Drawing of the anal canal in the coronal plane shows thesimple intersphincteric fistula to the left of the anal canal.
Grade 2: intersphincteric fistula with an abscess. (a) Axial drawing ofthe anal canal shows a right posterolateral abscess (arrow).
(b) Axial T2-weighted MR image shows the high-signal-intensity fluidcollection along the right posterolateral aspect of the anal canal(arrow).
(c) Axial contrast-enhanced fat-suppressed T1-weighted MR image shows theabscess in the right posterolateral aspect of the intersphincteric space(arrowhead), bounded by the external sphincter.
Grade 2: intersphincteric fistula with an abscess (same patient as in previous Fig). (a) Coronal drawing of the anal canal shows the abscess in theintersphincteric space (arrow), bounded by the external sphincter.
(b) Coronal contrast-enhanced fat-suppressed T1-weighted MR image showsthe right intersphincteric abscess (arrow) without a fistulous track or abscessin the right ischiorectal fossa.
Grade 3: transsphincteric fistula. (a) Axial drawing of the anal canalshows a posterior transsphincteric fistula (arrow) with the internalopening at the 6-o’clock position. (
(b) Axial contrast-enhanced fat-suppressed T1- weighted MR imageshows the transsphincteric fistula (arrow) crossing the externalsphincter.
Grade 3: transsphincteric fistula (same patient as in previous Fig ). (a)Coronal drawing of the anal canal shows the right transsphinctericfistula.
(b) Coronal contrast-enhanced fat-suppressed T1-weighted MR imageshows the highly enhancing transsphincteric fistula (arrow) from thedentate line to the skin, passing through the ischioanal fossa andpiercing the external sphincter.
Grade 4: transsphincteric fistula with an abscess or secondary track in theischiorectal or ischioanal fossa. (a) Axial drawing of the anal canalshows a posterior transsphincteric fistula with an abscess in the rightischiorectal fossa.
(b) Axial T2-weighted MR image shows the transsphincteric fistulacrossing the external sphincter at the 6-o’clock position (arrow) and ahigh-signal-intensity fluid collection in the right ischiorectal fossa(arrowheads).
(c) Axial contrast-enhanced fat-suppressed T1- weighted MR imageshows the posterior transsphincteric fistula (straight arrow), theabscess in the right ischiorectal fossa with nonenhancing pus in thecavity (arrowheads), and a secondary extension in the left ischiorectalfossa (curved arrow).
Grade 4: transsphincteric fistula with an abscess or secondary track in theischiorectal or ischioanal fossa (same patient as in previous Fig ). (a)
(b) Coronal contrast-enhanced fat-suppressed T1-weighted MR imageshows the abscess in the right ischiorectal fossa with nonenhancing pus inthe cavity (arrowheads) and the secondary extension in the left ischiorectalfossa (arrow).
Grade 5: supralevator and translevator disease. (a) Axial drawing of the analcanal shows a supralevator abscess located at the urethra (U), the left sideof the anal canal, and the left internal obturator muscle (IO).
(b) Axial contrast-enhanced fat-suppressed T1-weighted MR image showsthe left supralevator abscess with inflammatory changes in the left internalobturator muscle (arrows).
Grade 5: supralevator and translevator disease (same patient as in previous Fig). (a) Coronal drawing of the anal canal shows the left supralevatorabscess with a left translevator fistula.
(b) Coronal contrast-enhanced fat-suppressed T1-weighted MR imageshows the left supralevator abscess with inflammatory changessurrounding the rectum and the left translevator fistula crossing theischiorectal fossa (arrowheads).
Horseshoe abscess. Axial T2-weighted MR image shows a horseshoe abscess witha fluid-fluid level in both ischiorectal fossae (arrowheads). The abscess has highsignal intensity due to pus and a liquid-liquid level due to detritus.
Horseshoe abscess (same patient as in previosu Fig ). Axial (a) and coronal (b)contrast-enhanced fat-suppressed T1-weighted MR images show ahorseshoe abscess in the ischiorectal and ischioanal fossae (arrows ina, arrowheads in b). The abscess has intense enhancement due to thepresence of active inflammatory tissue.