Neuroimaging Mechanism & Clinical Approach PDF

Ocular Disease III: Neuro-Ophthalmic Disease Neuroimaging Hua Bi, OD, PhD, FAAO, Dipl AAO Diplomate, American Board of Optometry Why neuroimaging? Plain Radiography Based on the absorption of X-rays as they pass through different parts of the body. The X-rays are directed to the ap...

Ocular Disease III: Neuro-Ophthalmic Disease Neuroimaging Hua Bi, OD, PhD, FAAO, Dipl AAO Diplomate, American Board of Optometry Why neuroimaging? Plain Radiography Based on the absorption of X-rays as they pass through different parts of the body. The X-rays are directed to the appropriate area of the body. As the X-rays pass through the body they are attenuated by the tissues. Depending on the amount absorbed in a particular tissue, different amount of X rays will pass through and exit the body. The exiting X-rays interact with a detection device (X-ray film or other image receptor) and provide a 2-dimensional projection image of the tissues within the body. e.g. The Chest X-ray, Joint X-ray Contraindications: Pregnancy Computed Tomography (CT SCAN / CAT SCAN) The X-ray tube and the detector move around the patient and images can be acquired at different tissue levels. Multiple cross-sectional images are digitally constructed from density measurement by calculation of attenuation coefficients: The computer reconstructs the image from data points that are assigned a numerical value based on the attenuation of the X-ray beams Computed Tomography (CT SCAN / CAT SCAN) To display the values of each data point, gray scales are chosen DARK White HU = 1000 x (µtissue – µH2O)/ µH2O Note: μ is the CT linear attenuation coefficient Computed Tomography (CT SCAN / CAT SCAN) Denser tissues block more X-rays and the image is “brighter” on CT scan by convention: Brightness: Bone > Brain tissue > Water > Air Soft-tissue details can be further enhanced with the administration of iodinated contrast material Helical CT image acquisition: It is used in most modern CT protocols It allows the CT tube and the detector to continually rotate around the patient A three-dimensional data set is then generated, and be reconstructed into sequential images SCANNING Advantages over conventional scanning acquisition: § Shorter examination times FASTER § Reduction of motion artifacts BEHERImaging § Can rapidly acquire data for CT angiography Multidetector CT (MDCT) Indications for Use of CT § Complex fractures or those requiring surgical repair § Foreign body localization CTSCANUSE § Orbital lesions/ocular tumors: § hamartomas § choroidal osteoma § some metastatic tumors § Lesions causing bone loss or lesions causing calcification § Acute hemorrhagic lesions (e.g., subarachnoid or intraparenchymal hemorrhage) § Thyroid eye disease (especially if orbital surgery is being considered) § Patients with MRI contraindication or intolerance (e.g., claustrophobia) Risk: ionizing radiation!! Radiation exposure and cancer: See reference reading material Contraindications for CT 1. Pregnancy 2. Young age 3. For iodinated contrast CT: Sensitivity to iodine Previous reaction Bronchial asthma, respiratory dysfunction (increased risk of an allergic-like reaction) Renal dysfunction Diabetes Thyroid disease Multiple myeloma Sickle cell disease What are the Radiation Risks from CT? Carcinogenic and teratogenic effects Comparison of Radiation Doses From Medical Imaging Tests and Background Radiation * IF * These doses are effective doses, which are theoretical quantities proposed by the International Commission on Radiation Protection to assess the health risks of low doses of ionizing radiation. Mayo Clin Proc. 2010 Magnetic Resonance Imaging Mechanism of Operation The primary origin of the MR signal used to generate common clinical images comes from hydrogen nuclei. Hydrogen nuclei consist of a single proton that carries a positive electrical charge. All protons spin creating small magnetic moments, which are normally randomly orientated. When an external magnetic field is introduced in MRI machine, the protons align with that field. Then a radiofrequency (RF) pulse is used to tip the protons out of alignment with the external magnetic field: hydrogen "resonate" to radiofrequency by absorbing the radiofrequency energy. Once this pulse is turned off, the protons realign with the external magnetic field, releasing electromagnetic energy. This energy is detected and reconstructed by computer to produce an image. MRI is able to differentiate various tissues based on their magnetic properties. Longitudinal magnetization & transverse magnetization z y x Longitudinal magnetization Transverse magnetization The direction of tissue magnetization The direction of tissue magnetization is parallel to the direction of the is at a 90 degree angle with respect magnetic field to the direction of the magnetic field and is in the transverse plane. T1 relaxation T2 relaxation (spin-lattice relaxation): (spin-spin relaxation): describes how fast longitudinal describes how fast transversal magnetization recovers magnetization vanishes T1 relaxation time T2 relaxation time The order and timing of magnetic gradient application, radiofrequency pulse application, and radiofrequency recording characterizes the imaging acquisition sequence. T1- and T2-weighted images (T1WI and T2WI): the two most common MRI sequences used to enhance the contrast between tissues of different signal intensities T1-weighted image (T1WI): T2-weighted image (T2WI): demonstrates differences in demonstrates differences in the T1 relaxation times of the T2 relaxation times of tissues tissues Better for demonstrating Typically better for normal anatomy distinguishing pathology Signal intensity variation in T1 and T2 weighted images T1: CSF & vitreous is hypointense (dark); gray matter is relatively hypointense compared to white matter T2: CSF & vitreous is hyperintense (bright); gray matter is relatively hyperintense compared to white matter Imaging Terms: sequence parameters TR (repetition time): the time between successive RF pulses TE (echo time): the time interval between the beginning of transverse relaxation and when the magnetization is measured to produce image contrast. T1 Weighted: short TR/TE T2 Weighted: long TR/TE Proton Weighted (signal related to proton density): long TR with short TE  Gadolinium MRIcontrastmedium Gadolinium contrast medium in MRI signalintensitylocallymF fibrosisNSF MAYcause nephrogenicsystemic pkEfIaFgfIftaisehEfnel contradiction Gadolinium, a paramagnetic substance, is used as an MR contrast medium. It enhances the local magnetic field and increases signal intensity. Risk: Gadolinium-based contrast agents may cause nephrogenic systemic fibrosis (NSF), a multisystemic potentially fatal disease of the heart, lung, and liver characterized by soft-tissue collagen deposition that results in skin thickening and muscle contractures in patients with renal failure Contraindications: Pregnant women because the effects on the fetus are not known Patients with preexisting renal disease Recent kidney or liver transplants Severe renal failure or with concomitant liver failure  Flair SuppressfluidfromglowingUP Anywhere CSFislocated Specific suppression sequences: glow 4 demyelinatingdisease FLAIR (Fluid-attenuated inversion recovery) Suppress the hyperintense signal of CSF on T2WI The FLAIR sequence improves visualization of hyperintense lesions adjacent to CSF-filled space (e.g. periventricular white matter demyelination). May have to specifically ask for FLAIR sequences in suspected demyelinating disease Specific suppression sequences: Fat suppression: Fat suppression sequences are commonly applied to suppress the hyperintense signal of fat. allow better visualization of pathological hyperintensity allow confirming the content of fat-containing lesions, such as orbital dermoid cysts and lipomas Fat containinglesions EgFats Examples of method: Short inversion time inversion recovery (STIR) Fat saturation  Rapidmoleculardiffusionus slowdiff Diffusion-weighted imaging (DWI) TissueBoundtheMOREBounded diffusion Identifyrecentinfractionstoperfusion It is based upon measuring the Brownian motion of water molecules (molecular diffusion) within the tissue and provides contrast among tissues according to their diffusibility characteristics Tissue-bound water has restricted molecular diffusion compared to free water. Diffusion weighting imaging is able to distinguish between rapid diffusion of protons (unrestricted diffusion) and slow diffusion of protons (restricted diffusion). It is sensitive to recent alterations in vascular perfusion and is thus ideal for identifying recent infarctions (cerebrovascular accident). Diffusion-weighted imaging (DWI) showing acute ischemic infarcts that are bright on axial DWI MRI indications: § Optic disc edema or pallor § Optic neuritis § Orbital mass, thyroid-associated orbitopathy, orbital injury, orbital inflammation, asymmetry, exophthalmos, enophthalmos § Vision loss. optic nerve, pre-chiasm, chiasm or post-chiasm symptoms § Diplopia disorders of neural origin or ophthalmoplegia § Third, fourth, sixth nerve palsy or cavernous sinus syndrome § Nystagmus § Facial or lid disorders that have neural origin (e.g. in brainstem) § Acquired Horner’s syndrome § Headache, eye pain of neuro-ophthalmic causes Contraindications for MRI Incompatible medical implants/foreign bodies: Cardiac pacemakers Metallic prosthetic devices Intracranial metal vascular clips on vascular structures Old cardiac valve prosthesis Cochlear implants or neuro-stimulators All metallic objects present on patients such as necklaces, rings, watches, eye makeup MRI safety & pregnancy Intracranial vascular imaging: Catheter angiography aka digital subtraction angiography (DSA), conventional angiography Requires invasive arterial access Individual great vessels are identified and iodinated contrast media is infused Images are captured using ionizing radiation (x- rays) and then constructed by a technique called digital subtraction angiography Can be used for diagnosis and therapy Complications: Contrast media reactions Hematoma at the puncture site Vasospasm Emboli leading to ischemia COMPUTED TOMOGRAPHY ANGIOGRAPHY (CTA) CTA involves the use of ionizing radiation and iodinated contrast material. The contrast material is injected into a peripheral vein (typically antecubital vein). The IV injection of contrast material is followed by high-speed spiral CT scanning. COMPUTED TOMOGRAPHY ANGIOGRAPHY (CTA) Image display techniques: § Three-dimensional volume rendering § Maximum intensity projection Maximum intensity projection Three-dimensional volume rendering Contraindication: Pregnancy Severe iodinated contrast allergy or those who should not receive iodinated contrast agents MAGNETIC RESONANCE ANGIOGRAPHY (MRA) § Noninvasive § Does not utilize ionizing radiation or iodinated contrast material Indications: Evaluation of aneurysms vascular malformations, dissection, stenosis occlusion in transient ischemic attacks amaurosis fugax completed strokes MAGNETIC RESONANCE ANGIOGRAPHY (MRA) Types of MRA techniques: Time of flight (TOF) Based on magnitude effects Stationery tissue become magnetically saturated by multiple repetitive radiofrequency pulses Fresh inflowing blood, which traverse the region of interest, gives high initial magnetization signal TOF MRA Contrast-enhanced MRA MAGNETIC RESONANCE ANGIOGRAPHY (MRA) Types of MRA techniques: Phase contrast (PC) § Uses magnetic field gradients to induce phase shift in flowing blood § The phase shift is proportional to the velocity of blood § Allows for quantification of flow velocity and flow rate MAGNETIC RESONANCE ANGIOGRAPHY (MRA) Contrast-enhanced MRA Involves the intravenous injection of a gadolinium-based contrast medium The contrast medium shortens the T1 value of blood, which increases the intravascular signal and makes it less dependent on laminar flow. MRV (MR venography) Indications: Patients with venous diseases (e.g. venous sinus thrombosis, which increases intracranial pressure with resultant papilledema) MRV (MR venography) Contrast enhanced MRV reduces the incidence of artifacts seen with non-contrast MRV Diffusion tensor imaging (DTI) Generates exquisite structural images of brain white matter tracts via measurement of water molecule diffusion within white matter Anisotropy Functional Magnetic Resonance Imaging (fMRI) Blood-oxygen-level-dependent (BOLD) fMRI detects changes in the oxygenation state of hemoglobin, thereby capturing oxygen consumption associated with neuronal activation Magnetic resonance spectroscopy (MRS) Measures the concentrations of molecules associated with brain metabolism (such as N-acetyl aspartate (NAA), choline (Cho), creatine (Cr) and lactate) Positron emission tomography (PET) Uses radiolabeled metabolic analogs to measure the rate of brain glucose metabolism Comparison of MRI sequences T1 NORMAL T2 glowAround T2 with FLAIR MAKES tissuefluidspaces everything darker MRI sequences comparison T1 with FLAIR Comparison of MRI sequences T1 weighted T1 weighted with fat suppression Comparison of MRI sequences T2 weighted T2 weighted with fat suppression 4spreesFluid Reading the MRI scan Planes used in modern imaging procedures: View as you would face the patient, or from the patient’s feet upward 1. Axial: divide the body into superior and inferior parts 2. Coronal: oriented vertically and divide the body into anterior and posterior parts 3. Sagittal: oriented vertically and divide the body into right and left parts Axial Reading the MRI scan a b coronal c Axialplane plane Sagittalplane Brain MRI images obtained from Axial plane (a), Coronal plane (b) and Sagittal plane (c) Clinical Approach to Imaging Studies Ordering imaging studies: Images should be ordered after a complete history has been taken, a thorough examination performed, and differential diagnosis obtained. Select appropriate neuroimaging and the timing of the study based on the patient presentation and limitations Communication with neuroradiologist is important regarding suspected lesion, localization, image study of choice, scan thickness, sequence, contrast selection, indications  USEMRIfirst ifcontradictionofMRI then Indications for CT and MRI USE a metals prostheticv1 metals In general, MRI offers superior visualization of soft tissues and visual pathways. CT scan is not as sensitive or specific as an MRI image for orbital and brain imaging studies with important exception: CT is indicated for trauma, acute hemorrhage, fractures or calcifications within a mass lesion, MRI contraindications Both MRI and CT should be used for conditions where they may be complementary In suspected vascular disease, consider MRA, CTA, and CA Imaging of the orbit and head: Orbital imaging provides details about the optic nerves and the surrounding tissues that often are not detected with brain imaging alone. Therefore, an imaging of the head is not a substitute for an imaging of the orbit. Both coronal and axial sections are needed. Reviewing imaging studies: Always review any study that you have ordered Always review them yourself and clinically correlating each study yourself! Verify it’s the correct scan Patient, date, study requested Scan quality, orientation, scan thickness Identify normal reference structures and abnormal findings, use right/left comparisons if possible Make your differential diagnosis based on scan results and correlate to the clinical findings, refer for additional management or consultation if indicated If the imaging shows either no abnormality or an abnormality that does not match the clinical findings then, reexamine the clinical findings, reexamine the imaging study including suspected lesion, localization, image study of choice, scan thickness, orientation, sequence, contrast selection etc. Recognize that the lack of an imaging abnormality does not exclude pathology