Archive for 2015
Indications for MRI brain
Contraindications
Patient preparation for MRI brain
Positioning for MRI brain
Protocols and planning
Localiser
A three plane localizer must be taken in the beginning to localise and plan the sequences. Localizers are usually less than 25sec. T1 weighted low resolution scans.
T2 tse axial
Plan the axial slices on the sagittal plane; angle the position block parallel to the genu and splenium of the corpus callosum. Slices must be sufficient to cover the whole brain from the vertex to the line of the foramen magnum. Check the positioning block in the other two planes. An appropriate angle must be given in coronal plane on a tilted head (perpendicular to the line of 3rd ventricle and brain stem).
T2 FLAIR axial
Plan the axial slices on the sagittal plane; angle the position block parallel to the genu and splenium of the corpus callosum. Slices must be sufficient to cover the whole brain from the vertex to the line of the foramen magnum. Check the positioning block in the other two planes. An appropriate angle must be given in coronal plane on a tilted head (perpendicular to the line of 3rd ventricle and brain stem).
T1 SE coronal
Plan the coronal slices on the sagittal plane; angle the position block parallel to the brain stem. Check the positioning block in the other two planes. An appropriate angle must be given in the axial plane on a tilted head (perpendicular to mid line of the brain). Slices must be sufficient to cover the whole brain from the frontal sinus to the line of the occipital protubernce.
T2 tse sagittal
Plan the sagittal slices on the axial plane; angle the position block parallel to midline of the brain. Check the positioning block in the other two planes. An appropriate angle must be given in the coronal plane on a tilted head (parallel to the line along 3rd ventricle and brain stem). Slices must be sufficient to cover the brain from temporal lobe to temporal lobe.
DWI epi3scan trace axial
Plan the DWI axial slices on the sagittal plane; angle the position block parallel to the line from the glabella to the foramen magnum. This angle will reduce air-bone interface artefacts from the Para nasal sinuses. Slices must be sufficient to cover the whole brain from the vertex to the foramen magnum. Check the positioning block in the other two planes. An appropriate angle must be given in the coronal plane on a tilted head (perpendicular to the line of 3rd ventricle and brain stem).
Indications for contrast enhancement brain scans
Use T1 SE axial and coronal after the administration of IV gadolinium DTPA injection(copy the planning outlined above). The recommended dose of gadolinium DTPA injection is 0.1 mmol/kg, i.e. 0.2 mL/kg in adults, children and infants.
- Transient ischaemic attack (TIA), syncope, collapse , stroke
- Brain Tumour, Suspected brain tumour, metastases, papilloedema
- CNS infection, abscess, meningitis, AIDS,&TB
- Congenital malformation of brain or meninges
- Post-operative follow-up after brain surgery
- Dementia, neurodegenerative disorder
- Demyelinating disease of the brain
- Encephalopathy, encephalitis
- Cerebellar, or brainstem lesion
- Head trauma, epilepsy,stroke
- CVA, altered mental status
- Suspected leukodystrophies
- Ataxia, bipolar disorder
- Multiple sclerosis
- ENT problems
Contraindications
- Any electrically, magnetically or mechanically activated implant (e.g. cardiac pacemaker, insulin pump biostimulator, neurostimulator, cochlear implant, and hearing aids)
- Intracranial aneurysm clips (unless made of titanium)
- Pregnancy (risk vs benefit ratio to be assessed)
- Ferromagnetic surgical clips or staples
- Metallic foreign body in the eye
- Metal shrapnel or bullet
Patient preparation for MRI brain
- A satisfactory written consent form must be taken from the patient before entering the scanner room
- Ask the patient to remove all metal objects including keys, coins, wallet, cards with magnetic strips, jewellery, hearing aid and hairpins
- If possible provide a chaperone for claustrophobic patients (e.g. relative or staff )
- Contrast injection risk and benefits must be explained to the patient before the scan
- Gadolinium should only be given to the patient if GFR is > 30
- Offer earplugs or headphones, possibly with music for extra comfort
- Explain the procedure to the patient
- Instruct the patient to keep still
- Note the weight of the patient
Positioning for MRI brain
- Head first supine
- Position the head in the head coil and immobilise with cushions
- Give cushions under the legs for extra comfort
- Centre the laser beam localiser over the glabella
Protocols and planning
Localiser
A three plane localizer must be taken in the beginning to localise and plan the sequences. Localizers are usually less than 25sec. T1 weighted low resolution scans.
T2 tse axial
Plan the axial slices on the sagittal plane; angle the position block parallel to the genu and splenium of the corpus callosum. Slices must be sufficient to cover the whole brain from the vertex to the line of the foramen magnum. Check the positioning block in the other two planes. An appropriate angle must be given in coronal plane on a tilted head (perpendicular to the line of 3rd ventricle and brain stem).
T2 FLAIR axial
Plan the axial slices on the sagittal plane; angle the position block parallel to the genu and splenium of the corpus callosum. Slices must be sufficient to cover the whole brain from the vertex to the line of the foramen magnum. Check the positioning block in the other two planes. An appropriate angle must be given in coronal plane on a tilted head (perpendicular to the line of 3rd ventricle and brain stem).
T1 SE coronal
Plan the coronal slices on the sagittal plane; angle the position block parallel to the brain stem. Check the positioning block in the other two planes. An appropriate angle must be given in the axial plane on a tilted head (perpendicular to mid line of the brain). Slices must be sufficient to cover the whole brain from the frontal sinus to the line of the occipital protubernce.
T2 tse sagittal
Plan the sagittal slices on the axial plane; angle the position block parallel to midline of the brain. Check the positioning block in the other two planes. An appropriate angle must be given in the coronal plane on a tilted head (parallel to the line along 3rd ventricle and brain stem). Slices must be sufficient to cover the brain from temporal lobe to temporal lobe.
DWI epi3scan trace axial
Plan the DWI axial slices on the sagittal plane; angle the position block parallel to the line from the glabella to the foramen magnum. This angle will reduce air-bone interface artefacts from the Para nasal sinuses. Slices must be sufficient to cover the whole brain from the vertex to the foramen magnum. Check the positioning block in the other two planes. An appropriate angle must be given in the coronal plane on a tilted head (perpendicular to the line of 3rd ventricle and brain stem).
Indications for contrast enhancement brain scans
- Tumour, Metastases, Cranial nerve lesion, Indeterminate intracranial lesion, IAC mass
- Cavernous angioma, Amyloid angiopathy, Neurocysticercosis
- Meningitis, Encephalitis, Leptomeningeal spread
- Multiple Sclerosis, AVM, HIV, Infection Abscess
- Leukodystrophies, Delayed development
- Syringomyelia(Syrinx)
Use T1 SE axial and coronal after the administration of IV gadolinium DTPA injection(copy the planning outlined above). The recommended dose of gadolinium DTPA injection is 0.1 mmol/kg, i.e. 0.2 mL/kg in adults, children and infants.
The advantages of this technique are that it involves no ionising radiation, is capable of multi-planar imaging, affords high-contrast resolution (with more detailed evaluation of bowel wall changes) and allows for cine-imaging.
Its main indication at present is to evaluate small bowel involvement in patients with Crohn’s disease ( is a chronic inflammatory bowel condition with onset usually in young adulthood. Twenty to thirty percent of patients are younger than 20 years old)
Common uses of this procedure (MR enterography) to identify and locate
- The presence of and complications from Crohn's disease and other inflammatory bowel diseases
- Inflammation
- Bleeding sources and Vascular abnormalities
- Tumors
- Abscesses and Fistulas
- Bowel obstructions.
Main sequence used in MRE
Coronal True FISP image: normal bowel. The ‘black boundary’ artefact may be confused with bowel wall thickening (arrows)
Coronal T1 fat-saturated post-contrast image: normal bowel wall shows mild homogeneous enhancement
Coronal fat-saturated HASTE image: normal bowel. Intraluminal flow voids (arrow) are seen, as this sequence is sensitive to fluid motion.
Example of mural thickening in active Crohn’s disease: a axial True FISP image shows mural thickening in the distal ileum (arrow); b coronal True FISP in a different patient (15 years old) shows extensive jejunal small bowel wall thickening
Example of pseudosacculation: coronal True FISP image shows two long skip lesions in the mid to distal ileum (asterisk) demonstrating mural thickening, luminal narrowing and prominence of the vasa recta (arrowheads) with small nodes seen within the mesentery.
Atelectasis and tumor. On MRI a big mass in the left upper lobe is identified. Superior to this mass there is an area of increased signal intensity. This might well be an atelectasis. In the axial plane the differentiation of atelectasis and tumor is not possible. Neither MRI nor CT can differentiate atelectasis from tumor with confidence.
Reoccurrance of a bronchial carcinoma. Post-resection treatment on the left side. 12 months later, soft-tissue formation on the bronchial stump with infiltration in the mediastinal fatty tissue and walling of the bronchial vasculature.
Low-malignant Non-Hodgkin-Lymphoma. Bilateral, segmental infiltrates in the lingula and middle lobe with a positive bronchogram. The thoracic overview shows a widened left hilus
Large liposarcoma in the left 6th segment
Lipoma in the left dorsal lateral sinus. Lipomas develop transcompartimentally through gaps
Magnetism
Magnetism is a property of matter that is a result of the orbiting electrons in atoms. The orbiting electrons cause the atoms to have a magnetic moment associated with an intrinsic angular momentum called 'spin'. Magnetic field strengths are measured in units of gauss (G) and Tesla (T). One Tesla is equal to 10,000 gauss. The earth's magnetic field is about 0.5 gauss. The strength of electromagnets used to pick up cars in junk yards is about the field strength of MRI machines (1.5-2.0T). You will run across four terms describing the magnetic properties of materials, such as contrast agents, used in MRI. These terms are
Ferromagnetism
Paramagnetism
Superparamagnetism
Diamagnetism
Resonance and RF
Protons in a magnetic field have a microscopic magnetization and act like tiny toy tops that wobble as they spin.The rate of the wobbling or precession is the resonance or Larmor frequency. In the magnetic field of an MRI scanner at room temperature, there is approximately the same number of proton nuclei aligned with the main magnetic field Bo as counter aligned. The aligned position is slightly favored, as the nucleus is at a lower energy in this position. For every one-million nuclei, there is about one extra aligned with the Bo field as opposed to the field. This results in a net or macroscopic magnetization pointing in the direction of the main magnetic field. Exposure of individual nuclei to RF radiation (B1 field) at the Larmor frequency causes nuclei in the lower energy state to jump into the higher energy state.
On a macroscopic level, exposure of an object or person to RF radiation at the Larmor frequency, causes the net magnetization to spiral away from the Bo field. In the rotating frame of reference, the net magnetization vector rotate from a longitudinal position a distance proportional to the time length of the RF pulse. After a certain length of time, the net magnetization vector rotates 90 degrees and lies in the transverse or x-y plane. It is in this position that the net magnetization can be detected on MRI. The angle that the net magnetization vector rotates is commonly called the 'flip' or 'tip' angle. At angles greater than or less than 90 degrees there will still be a small component of the magnetization that will be in the x-y plane, and therefore be detected.
Relaxation
T1 Relaxation
The return of excited nuclei from the high energy state to the low energy or ground state is associated with loss of energy to the surrounding nuclei. Nuclear magnetic resonance was originally use to examine solids in the form of lattices, hence the name "spin-lattice" relaxation. Macroscopically, T1 relaxation is characterized by the longitudinal return of the net magnetization to its ground state of maximum length in the direction of the main magnetic field. The rate of return is an exponential process as is shown in the following figure.
The T1 relaxation time is the time for the magnetization to return to 63% of its original length. After two T1 times, the magnetization is at 86% of its original length. Three T1 times gives 95%. Spins are considered completely relaxed after 3-5 T1 times. Another term that you may hear is the T1 relaxation rate. This is merely the reciprocal of the T1 time( 1/T1). T1 relaxation is fastest when the motion of the nucleus (rotations and translations or "tumbling rate") matches that of the Larmor frequency. As a result, T1 relaxation is dependent on the main magnetic field strength that specifies the Larmor frequency. Higher magnetic fields are associated with longer T1 times.
T2 Relaxation
Microscopically, T2 relaxation or spin-spin relaxation occurs when spins in the high and low energy state exchange energy but do not loose energy to the surrounding lattice. This results macroscopically in loss of the transverse magnetization. In pure water, The T2 and T1 times are approximately the same, 2-3 seconds. In biological materials, the T2 time is considerably shorter than the T1 time. For CSF, T1=1.9 seconds and T2=0.25 seconds. For brain white matter, T1=0.5 seconds and T2=0.07 seconds (70 msec). T2 relaxation occurs exponentially like T1 relaxation with 63% of the transverse magnetization gone after one T2 period as shown in the graph.
T2* Relaxation
T2* relaxation is the loss of signal seen with dephasing of individual magnetizations. It is characterized macroscopically by loss of transverse magnetization at a rate greater than T2. It is caused by magnetic field inhomogeneity an occurs in all magnets. The relationship between T2 and T2* can be illustrated by the multiecho spin echo sequence shown in the diagram below. The 180 degree RF pulses used to generate the echo are rephasing the spins that have undergone T2* decay. The gradual decline in signal from subsequent echos reflects T2 decay (See Figure). Unlike spin echo sequences, gradient echo sequences do not refocus T2* decay. Therefore, gradient echo sequences are more susceptible to ferromagnetic foreign bodies that distort the main magnetic field homogeneity
Computed Tomography
(CT scan) or Computed Axial Tomography (CAT scan), is a medical imaging procedure that utilizes computer-processed X-rays to produce tomographic images or 'slices' of specific areas of the body. These cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. CT produces a volume of data that can be manipulated, through a process known as "windowing", in order to demonstrate various bodily structures based on their ability to block the X-ray beam. Although historically the images generated were in the axial or transverse plane, perpendicular to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures. Although most common in medicine, CT is also used in other fields, such as non-destructive materials testing. Another example is archaeological uses such as imaging the contents of sarcophagi. Usage of CT has increased dramatically over the last two decades in many countries. An estimated 72 million scans were performed in the United States in 2007. It is estimated that 0.4% of current cancers in the United States are due to CTs performed in the past and that this may increase to as high as 1.5-2% with 2007 rates of CT usage; however, this estimate is disputed. Kidney problems following intravenous contrast agents may also be a concern in some types of studies.
(CT scan) or Computed Axial Tomography (CAT scan), is a medical imaging procedure that utilizes computer-processed X-rays to produce tomographic images or 'slices' of specific areas of the body. These cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. CT produces a volume of data that can be manipulated, through a process known as "windowing", in order to demonstrate various bodily structures based on their ability to block the X-ray beam. Although historically the images generated were in the axial or transverse plane, perpendicular to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures. Although most common in medicine, CT is also used in other fields, such as non-destructive materials testing. Another example is archaeological uses such as imaging the contents of sarcophagi. Usage of CT has increased dramatically over the last two decades in many countries. An estimated 72 million scans were performed in the United States in 2007. It is estimated that 0.4% of current cancers in the United States are due to CTs performed in the past and that this may increase to as high as 1.5-2% with 2007 rates of CT usage; however, this estimate is disputed. Kidney problems following intravenous contrast agents may also be a concern in some types of studies.
