Presentation on theme: "INTRODUCTION TO NEURO MR"— Presentation transcript:
1INTRODUCTION TO NEURO MR Dr. Francis NeufferUSC School of MedicineDepartment of Radiology
2MR IMAGINGBased on behavior of protons exposed to a magnetic field and a radio waveT1, T2, FLAIR, Diffusion, Gadolinium enhanced, and Angiography are specific types of Neuro imaging sequences.Sections are acquired by varying magnetic fields and pulsing radiowaves. The radiowaves disturb the alignment of the hydrogen protons in the magnetic field. The disturbed protons resonate and emit a characteristic radio wave which can be used for image generation. Varying the radio pulse time and the receiver time produces differing scans based on different tissue characteristics.
3This is an image of a typical MR scanner with a superconducting magnet. 3
4The key features in this diagram are the superconducting magnet which creates the stong magnetic field that aligns the protons. The radio frequency coil is a transmission and receiver coil to excite the sample tissue with radio frequency pulse and pick up the emitted signal once the excitatory pulse has terminated. The Gradient power adjusts the magnetic field to specific areas and the computer work station collects and displays the data.4
5RFThis is a diagram of a patient within the magnetic field with the protons aligned in the field and showing the RF pulse being transmitted into the sample. The RF pulse disturbs the alignment of the protons and this then causes the protons to resonate and emit an RF pulse which is the source of the signal.5
6MR INTERPRETATION Symmetry Identify normal structures Ventricles Grey matter structuresWhite matter tractsLanguage – Intensity/ SignalDescription of tissue signal on various different scanning sequences ie.T1 T2 Flair Diffusion GadoliniumThere will be a description of the technique used for the scan. The report will identify symmetry or the lack of it as an indicator of disease. Normal structures are identified. Regions of interest are described by their intensity of signal relative to normal structures. There is no numerical scale of density values as in CT. Areas are hyperintense, isointense, or hypointense and are described by their signal and the specific type of MR sequence obtained.
7MAGNETIC RESONANCE IMAGING Limitations1. ICU patients and Claustrophobia2. Metal artifact1. RF Energy – pacemaker override2. Magnetic field - aneurysm clips - ocular metal-missile effect3. Nephrogenic Systemic Fibrosis-gadolinium toxicity in renal failureICU patients have multiple electronic devices which are not compatible in the scanner. The patients are more likely unstable and cannot remain still for the protracted exam.( 30 minutes typical for a exam with multiple sequences). Pacemaker and metalic bodies are associated with problems as noted. An MR specific contrast Gadolineum can be used to evaluate the Blood/Brain barrier. This can help identify pathology and is handled physiologically similar to Iodine. The allergic and Renal issues are generally less than with Iodine contrast in CT although recent toxicity of Gadolineum in Renal failure patients is linked to a serious skin disorder Nephrogenic Systemic Fibrosis.(NSF)7
8A child undergoing an MRI exam received a Child Dies in MRI Machine The Associated Press Monday, July 30, 2001: 2:42 p.m. EDTA child undergoing an MRI exam received afatal head wound when the machine’s powerful magnet pulled a metal oxygen canister inside.Metallic objects can behave as missiles in the magnetic field.8
9GADOLINEUM TOXICITYGoogle searches show the attorneys are interested at least.9
10MAGNETIC RESONANCE IMAGING AdvantagesMultiple signal sourcesNo iodine toxicity/allergy issuesNo ionizing radiation issuesThese are advantages10
11MR HAS ADVANTAGE OF MULTI PLANAR IMAGING The patient is supine on the MR scanner but any plane of section can be obtained
12MRI INDICATIONS Ischemia Tumor Infection Dating blood products Congenital abnormalitiesThese are typical indications for MR scan
13MRI INTERPRETATION Pulse Sequences- T1 weighted-- (Fat, Melanin, Hemosiderin, Methemoglobin= bright)T2 weighted-- (Water, Oxyhemoglobin, Hemosiderin= bright)FLAIR-- (Pathology bright, CSF dark)Diffusion Weighted- recent infarction brightPulse sequences describe the type of MR scan that is obtained. They are determined by the time used to sample the tissue after the exciting radiowave has been emitted and how quickly a repeat radiowave is used to reexcite the tissue for a subsequent pulse.They are variously described as T1 weighted, T2 weighted, FLAIR, Diffusion etc.
14MR SIGNALT1 SCANT2 SCANTissues have intrinsic parameters classed as T1 and T2 values which describe how the tissue responds to the MR signal. The T2 identifies how long a tissue resonates after a pulse. The T1 identifies how quickly a tissue recovers and realigns in the magnetic field after the radio pulse ends -. These are characteristic of tissues and function as the basis for the imaging sequence. Varying the pulse time/delay/repetition gives an image with varying T1 and T2 effects.Tissues resonate a signal based on their intrinsic T2 time.Tissues recover their magnetization based on intrinsic T1 time.
15T1 SCAN T2 SCAN Anatomic structures Fat = bright Water = hypo intense The T1 scan shows anatomy better and the T2 scan shows pathology better. The T2 scan gets the brightest signal from water either free water in CSF or edema water in pathology. Note the Rt periventricular lesion is clearly visible on the T2 weighted scan. This ischemic infarct has edema and is poorly visualized on the T1 weighted scan.Water weighted sequenceWater = brightFat = relatively hypo intenseGood for identifying pathology
16MRI FINDINGS OF ACUTE ISCHEMIC STROKE T1 (hypo intense)T2 (hyper intense)FLAIR (hyper intense)The FLAIR scan stands for Fluid Attenuated Inversion Recovery. Basically the T2 water signal that comes from the free water (CSF) has been nulled so that the only bright T2 signal comes from edema. This makes visualization of pathology easier.FLAIR SCANS ARE T2 SCANS WITH THE FREE WATER SIGNAL NULLED.16
17MRI FINDINGS OF ACUTE STROKE T1 (hypo intense)T2 (hyper intense)Here the same patient has a diffusion scan as well. This is of value because the T2 weighted scan and the FLAIR will not separate acute infarction from chronic infarction. The Diffusion scan helps in this.FLAIR (hyper intense)Diffusion (hyper intense)17
18DIFFUSION IMAGINGDIFFUSION IMAGING SEPARATES INFARCTION ON ACUTE OR CHRONIC BASISTHE ACUTE INFARCT HAS A DIFFERENT DIFFUSION SIGNAL DUE TO INTRACELLULAR EDEMA18
19DIFFUSION IMAGING Increased sensitivity for early changes of edema Becomes abnormal within 30 mins.Ischemia Cellular Dysfunction Increased Intracellular Space Restricted Diffusion of Water Increased signalDistinguish b/w old and new strokeNew stroke = bright on DWI (diffusion weighted image)Old stroke (encephalomalacia) = low SI on DWIWith the onset of ischemia, there is failure of the Na/K pump and water flows into the cell from the extracellular space. This disturbs the overall balance of intracellular and extracellular water. The MR scan can pick up this difference since the water within the cell has more restricted diffusion due to interactions with intracellular proteins than does the extracellular water. This restriction of diffusion affects the MR signal of the water and allows for separation of tissue on the MR Diffusion sequence. The effect with ischemia occurs quickly and fades after several days with cell membrane lysis.
20MRI ACUTE STROKE T1 T2 Diffusion PCA DISTRIBUTION Another example of a large posterior cerebral infarctionT2DiffusionPCA DISTRIBUTION
21MRI OLD -VS- NEW ISCHEMIC INFARCT On this T2 weighted scan areas are hyperintense in each hemisphere. The diffusion scan shows the acute infarct is on the left and the older infarct in on the right.T1T2DIFFUSION21
22ANATOMY Temporal lobe Pons Middle Cerebellar peduncle 4th ventricle These 5 sections have labelled structures of the brain noted. The sagittal MR is a reference of the level of the plane of sectionTemporallobeBasillarartery4th ventriclePonsOccipitallobe4th ventricle
28CT WITH CONTRAST ENHANCEMENT ENHANCING RING LESIONStandard CT scans with and without Iodinated contrast identify pathology. MR with gadolineum is used similarlyNON ENHANCED
29MR T1- SCAN – WITHOUT GADOLINEUM T1 – SCAN WITH GADOLINEUM T2- SCAN T2 The T1 weighted scan is performed with and without gadolineum which highlights the posterior left occipital lesion.MRT2
30PITUITARY ADENOMA NORMAL ABNORMAL Here is a normal and an abnormal T1 sagittal scan of the sella.ABNORMAL
31T1 SAGITTAL MR WITHOUT GADOLINEUM WITH GADOLINEUM Note the intense enhancement on the T1 weighted scan with gadolineumWITHOUT GADOLINEUMWITH GADOLINEUM
32ACOUSTIC NEUROMA WITHOUT GADOLINEUM WITH GADOLINEUM Here an acoustic neuroma at the internal auditory meatus is shown on T1 weighted scansWITHOUT GADOLINEUMWITH GADOLINEUM
33VASCULAR ANATOMY MR ARTERIOGRAM Anterior cerebral Middle cerebral CavernousCarotidBasilararteryICAECACarotidbulbOn the left is a standard contrast carotid arteriogram. Iodinated contrast has been injected into the common carotid artery from a catheter introduced via femoral artery puncture. The common carotid, internal carotid and external carotid arteries are visible (CCA/ICA/ECA).On the right is a MR arteriogram produced from a venous injection of Gadolineum with a rapid MR scan obtained as the contrast flows through the arterial tree. The gadolineum increases the T1 signal greatly and can show normal and abnormal vascular flow without the need for an arterial injection.ECAVertebralarteryICACCACCA
34VASCULAR ANATOMY MCA ECA ICA TIME OF FLIGHT MRA TOF-MRA Basilar AnteriorcerebralMCATIME OF FLIGHT MRATOF-MRABasilararteryCavernouscarotidECAICABAMCAVertebralAn MR arteriogram can also be obtained without injection of any contrast. On the left is a Time of Flight MRA. In this case the contrast comes from the blood that has flowed into the volume of tissue. The original blood that was in the volume has flowed out of the slab of tissue and blood that was not magnetized has come into the volume. This forms the basis for the contrast in the scan where tissue can be separated by having magnetic signal or not. Note the small diseased MCA(red arrow). The scan on the right is the previous venous injected gadolineum MRA.ACACavernouscarotid
35VASCULAR ANATOMY Circle of Willis ACA MCA ACoA ICA PCoA Basilar Artery PCAThis is a diagram of the Circle of Willis at the skull base. MCA-middle cerebral artery ACA- anterior cerebral artery, PCA - posterior cerebral artery ACoA- anterior communicating artery PCoA- posterior communicating artery ICA –internal carotid arteryBasilar ArteryCircle of Willis
36VASCULAR DISTRIBUTIONS ACAThese diagrams show the relative vascular territtories of the anterior/middle/and posterior cerebral arteries outlined on axial diagramsMCAPCA
37STROKE Ischemic Hemorrhagic Large artery atherosclerosis Higher mortalityCardioembolismRecurrenceSmall vessel ischemiaOcclusion of small end arteriesPredilection for BG, IC, Pons, Corona RadiataHemorrhagicStroke is commonly separated by Ischemic 85% and Hemorragic 15%
38IMAGING OF STROKE CT MRI Better identification of acute hemorrhage AvailabilityDecrease expenseDecrease timeLess contraindicationsMRIMore sensitive to early changes of stroke ie. edemaIn the ED setting CT remains the primary screen based on consideration of hemorrhage
39CT FINDINGS SAHAcute blood products are easy to visualize on CT scanning. MR imaging is more problematic since the blood will have a complex signal depending on the oxygen state of the hemoglobin molecule and the state of the RBC membrane.This is especially so with subarachnoid hemorrhage. It can be very hard to see on MR and would be catastrophic if missed. This is one reason why CT remains a primary Neurologic screen.
40STROKE INTERVENTIONTHROMBOLYTIC THERAPY TO SALVAGE ISCHEMIC BRAIN AT THE BORDER OF THE INFARCT ZONEWHO BENEFITS AND HOW SELECT?After there is proof of ischemic brain attack what are options?40
41STROKE INTERVENTION TIME CONSTRAINTS 3-6 HOUR WINDOW RISK OF HEMORRAGIC CONVERSIONWITH THROMBOLYTIC THERAPY41
42LTRTHere a patient has an occluded Rt MCA and no hemorhage on the CT scan. There is some edema in the rt hemishere based on sulcal effacement.There is occlusion of flow in the Rt MCA with a normal Lt MCA42
43An interventional procedure removed thrombus in the RT MCA and restored blood flow. 43
44The ischemic tissue bled due to arterial perfusion being restored into the affected zone creating a worse situation than the original ischemic infarct. Is there a way to predict who would benefit from aggressive thrombolysis?44
45Recent articles are directed toward trying to identify the relative size of the infarct zone and the ischemic zone to decide who would benefit.45
46MR PERFUSION Gadolineum Injection With MR diffusion imaging and gadolineum injection the size of the infarct zone (diffusion) and the ischemic zone( gadolineum) can be determined. If there is a large ischemic zone beyond the infarct territory these patients may benefit from therapy. If the zones are of similar size, the value is lessened from aggressive treatment. This is being actively researched and data is incomplete.46
47CT PERFUSION Iodine Injection CT scanning has similar possibilities for evaluation of this and is a subject of research as well.47
48GOAL FOR IMAGINGCOMPARISON OF INFARCT ZONE AND ISCHEMIC ZONE TO IDENTIFY TREATMENT CANDIDATES48
49MRI FINDINGS OF ACUTE STROKE T1 (hypointense)T2 (hyperintense)To conclude, the MR exam is a series of scans with different scanning sequences to evaluate tissue. Ischemic tissue will have more water and have a decreased signal on T1 approaching the water signal of CSF. It will have increased signal on T2 for the same reason. The FLAIR image is a T2 scan with the free water signal nulled showing the edema. The diffusion signal shows the area of restricted diffusion due to intracellular edema and indicates a acute infarction. It is the group of the 4 sequences that together indicate the pathology and not a single scan sequence.FLAIR (hyperintense)Diffusion (hyperintense)49