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MRI Basics

Unlike CT, which requires ionising radiation, MRI is based on the interaction between radio waves and hydrogen nuclei in the body in the presence of a strong magnetic field. There is no ionising radiation and no known safety issues. MRI produces higher quality images than CT. In CT scans must be in the plane of the gantry, that is, axial or semi-coronal. In MRI, one is able to acquire images directly in any plane, that is, the usual axial, sagittal, coronal, or any other.

Both modalities focus on the properties of a volume element or "voxel" of tissue. This is represented in 2-D form or picture elements or "pixels". The pixel intensity in CT reflects the electron density but in MRI it reflects the density of hydrogen, generally as water (H20) or fat. To be more exact, MR signal intensity reflects the density of mobile hydrogen nuclei modified by the chemical environment, that is, by the magnetic relaxation times, T1 and T2, and by motion.

A hydrogen nucleus is a single proton. It has a positive charge and spins and so generates a small magnetic field (a "magnetic moment") which are usually randomly distributed. These magnetic moments align when placed in a larger magnetic field. With MRI the magnetic field across the body-sized sample is intentionally made non-uniform by superimposing additional magnetic field gradients that can be turned on and off rapidly. Activation of these additional magnetic fields results in a net gradient in the strength of the magnetic field across the body which is necessary for spatial localisation and imaging. The essential components of an MR imaging system include a large magnet which generates a uniform magnetic field, smaller electromagnetic coils to generate magnetic field gradients for imaging and a radio transmitter and receiver and its associated transmitting and receiving antennae or coils. In addition to these fundamental components, a computer is necessary to coordinate signal generation and acquisition and image formation and display.

When the body lies within a strong magnetic field, it becomes temporarily magnetised. This state is achieved when the hydrogen nuclei in the body align with the magnetic field. When magnetised, the body responds to exposure to radiowaves at a particular frequency by sending back a radiowave signal called a "spin echo". This phenomenon (NMR) only occurs at one frequency (the "Larmor frequency") corresponding to the specific strength of the magnetic field. The spin echo signal is composed of multiple frequencies, reflecting different positions along the magnetic field gradient. When the signal is broken into its component frequencies (by a technique called a "Fourier Transform"), the magnitude of the signal at each frequency is proportional to the hydrogen density at that location, thus allowing an image to be constructed. Thus, spatial information in MRI is contained in the frequency of the signal, unlike X-ray-based imaging modalities such as CT.

MRI with Gadolinium

This is the MRI equivalent of CT with contrast and uses Gadolinium which shortens T1 relaxation times. It is useful when there is suspicion of neoplastic, inflammatory lesions or abscesses. Also useful for detecting meningeal disease. It is not often used in acute stroke unless the diagnosis is in doubt.

MR Angiography

MRA is a useful non invasive method to study the cerebral vasculature. It works by enhancing the signal form moving blood and suppressing the signal from stationary tissue. There are two main types of process, Time of flight (TOF) and phase contrast (PC), neither of which require any contrast to be given. Lastly there is contrast enhanced MRA in which gadolinium is given.

Time of Flight MRA is dependent on the flow and the movement of the protons in the blood through the imaging plane. The tissue is first bombarded with RF pulses and the fresh moving blood entering the slice retains its signal intensity and contrasts markedly with background tissue. There may be some overestimation of the length and severity of a stenotic lesion due to technical reasons.It can be sued for both venous and arterial flow.

Phase contrast MRA involves taking 2 images directly after each other using opposite gradients and then subtracting one from the other with the flowing blood being the difference. It tends to used more in cardiac and thoracic aorta imaging.

If Gadolinium is given then Contrast enhanced MRA can be done and is similar in many ways to CTA only gadolinium is given and this enhances intravascular signal and scan acquisition is done such that arteries are imaged at the point of peak intravascular opacification. Gadolinium causes shortening of the T1 relaxation of blood compared with background tissue leading to the high signal intensity of blood on T1-weighted sequences. It is highly accurate and reproducible and can be done much quicker than the non invasive techniques. Flow related artefacts are few. The main issue is giving Gadolinium. Gadolinium chelate agents do have minor side effects. They can cause severe anaphylaxis and renal dysfunction. They are generally considered safe in patients even with abnormal renal function. There is a small risk of nephrogenic systemic fibrosis. In practice CE MRA has similar accuracy to Cerebral DSA. Again one of the major problems is an overestimation of stenosis which can be significant in selecting patient for potential surgery.

The indications for MRA are the same as those for CTA and DSA. These are
  • Identification of Intracranial and extracranial vessel narrowing or occlusion
  • Identification of and quantification of Carotid stenosis often to support Ultrasound findings
  • Identification of Berry Aneurysms
  • Identification of Vertebrobasilar or Carotid dissection
  • Identification of Arteriovenous malformations
  • Identification of Vasculitis with vascular beading, narrowing, obstruction
  • MR cerebral perfusion studies often combined with DWI

MR Venography

Investigation of choice in suspected Cerebral Venous sinus thrombosis. Can show obstructed sinus and thrombosis within and absence of flow

Relaxation times

  • T1 relaxation time: Time taken for 63% of longitudinal magnetisation to recover in a tissue. T1 is short in fat and long in water and proteins. Contrast causes T1 shortening.
  • T2 relaxation time: Time taken for 63% of transverse magnetisation to be lost in a tissue. Liquids have long T2 and large molecules a short T2.
  • T2* based on T2 decay and dephasing due to inhomogeneities in the magnetic field. Relevant in Gradient echo imaging.
  • Weighting T1 and T2 weighting can increase tissue contrast

Scan Sequences

  • T1 weighted: Dark : CSF and anything with increased water (oedema, tumour, infection, infraction, haemorrhage or flow void or calcification). Bright: Fat, subacute haemorrhage, melanin, protein rich fluid, slow flowing blood, gadolinium, laminar necrosis of an infarct. White matter is brighter than grey. Myelin is light grey. Grey matter is grey. T1 is better for showing anatomy. An acute stroke will be hypointense.
  • T2 Weighted:Dark: Calcification, Blood products, protein rich fluid, flow void. Bright: Anything with increased water - CSF, Oedema, tumour, Infarct, inflammation, infection, subdural collection, methaemoglobin in subacute bleed. CSF is white and brain tissue is darker and more intermediate. Fat is dark. Myelin is dark grey. Grey matter is brighter than white matter. Good for identifying pathology. T2 is bright due to water or any oedema in pathology. For example a fresh infarct with oedema will show up bright or hyperintense after a few days becoming most obvious after a few months. T2 weighted hypeintensities may be seen within 6 hours and are present in 90% by 24 hours.
  • FLAIR: Fluid attenuated inverse recovery. FLAIR scans are T2 scans with the free water signal nulled. CSF is now dark. Useful to see oedema and periventricular lesions which appear bright. An acute ischaemic stroke will be hyperintense.
  • Gradient echo (GRE): Excellent at identifying areas of blood and haemosiderin deposition such as in macrophages around an old bleed. Also useful in identifying microbleeds. Sensitivity for blood approaches CT within first 24 hours of a bleed. After several days GRE is the more sensitive modality.
  • Diffusion Weighted Imaging: Identifies areas where brownian motions is restricted due to cytotoxic cell death and very useful for identifying early ischaemic stroke. Ischaemic lesions appear bright even within 30 minutes. A new stroke is bright but an old stroke will have low signal intensity on DWI. The DWI is initially hyperintense and then gradually fades after 10-15 days when the lesion will be best seen on T2 and FLAIR.
  • Apparent Diffusion Coefficient: (ADC map) Used with DWI. Ischaemic lesions appear dark.If bright this may suggests the DWI increased signal changes are due to T2 shine through and old i.e. false positive.
  • Gadolinium Enhancement: Identifies pathology in which there is breakdown of the blood brain barrier. Also useful in producing an angiogram. Tumours or other lesions may show ring like enhancement. T1 with Gadolinium will show increased signal with a pituitary tumour, acoustic neuroma or meningioma.
Contraindications to MRI
  • Cardiac Pacemaker or AICD
  • Insulin pumps, neurostimulators, cochlear implants, etc. may be de-programmed
  • Brain aneurysm clips - check with manufacturer
  • Extreme claustrophobia
  • Need for monitoring - difficult in acutely ill
  • Relative C/I in early pregnancy- data lacking
  • Ocular or other metallic foreign bodies (skull X-ray can help to exclude)
  • Deep brain stimulator
  • Swan-Ganz catheter
  • Bullets or gunshot pellets near great vessels or vital organs such as lungs, heart or brain

Metal outside the brain and eye is NOT an absolute contraindication: Magnetic deflection is minimal compared to normal physiologic forces. Cardiac valves), inferior vena cava filters, biliary and vascular stents, IUD's and metallic prostheses are safe, unless there is doubt as to positional stability. MRI Policies - Safety and Contraindications . Information is changing all the time. Another useful site is MRI safety.com

Next: >> Stroke Imaging : MRI Basics part 2


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