Makindo Medical Notes

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MRI Basics

• Unlike CT, which uses 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 involved, making MRI a safer option with no known radiation-related safety issues.
• MRI produces higher quality images compared to CT. While CT images are typically acquired in the plane of the gantry (axial or semi-coronal), MRI can acquire images in any plane, including axial, sagittal, coronal, or any oblique angle.
• Both CT and MRI focus on the properties of a volume element or "voxel" of tissue, represented in 2-D as "pixels." In CT, pixel intensity reflects electron density, whereas in MRI, it reflects the density of hydrogen, primarily as water (H₂O) or fat. More precisely, MR signal intensity is influenced by the density of mobile hydrogen nuclei, which is further modified by the chemical environment, including magnetic relaxation times (T1 and T2) and motion.
• The hydrogen nucleus, which is a single proton, possesses a positive charge and spins, creating a small magnetic field ("magnetic moment"). Normally, these magnetic moments are randomly distributed. However, when placed in a strong external magnetic field, they align. MRI uses additional magnetic field gradients, which can be switched on and off rapidly, to intentionally create non-uniformity across the magnetic field, aiding in spatial localisation and imaging.
• An MRI system includes several key components:
  • A large magnet that generates a uniform magnetic field.
  • Smaller electromagnetic coils to generate magnetic field gradients for imaging.
  • A radio transmitter and receiver, with associated antennae or coils.
  • A computer to coordinate signal generation, acquisition, and the formation and display of images.
• When a body is placed in a strong magnetic field, it becomes temporarily magnetised as hydrogen nuclei align with the magnetic field. In this magnetised state, the body responds to radio waves of a specific frequency (the "Larmor frequency") by emitting a radio wave signal called a "spin echo." This signal is composed of multiple frequencies, reflecting different positions along the magnetic field gradient.
• The spin echo signal can be broken down into its component frequencies using a "Fourier Transform," with each frequency proportional to the hydrogen density at its location. This allows the construction of detailed images. Unlike X-ray-based imaging modalities such as CT, MRI encodes spatial information in the frequency of the signal.

Challenges of MRI Scanning

• MRI brain scans typically require the patient to lie within a confined space for 10 to 30 minutes, making it challenging for those with claustrophobia. Access to the patient during the scan is limited, and the process is noisy, complicating communication with radiographers.
• Some patients may not tolerate the process due to discomfort or anxiety, and while open MRI scanners exist for claustrophobic patients, they are less accessible and not available in all settings.
• It’s often necessary to balance clinical need with practicality, selecting the shortest MRI protocol that can provide the essential information required for diagnosis.

Examples

T2 FLAIR	T2
DWI	ADC
SWI	T1 Sagittal

MRI Basics

• MRI uses radio waves and strong magnetic fields to interact with hydrogen nuclei in the body, without ionising radiation, making it a safer alternative to CT. Unlike CT, MRI can acquire images in any plane (axial, sagittal, coronal, or oblique) with high resolution.
• Both MRI and CT evaluate volume elements or "voxels" of tissue, represented as "pixels" in 2D images. In CT, pixel intensity reflects electron density, while in MRI, it reflects hydrogen density, particularly in water and fat. MRI signal intensity depends on hydrogen nuclei density and is influenced by T1 and T2 relaxation times and tissue characteristics.
• MRI relies on the alignment of hydrogen nuclei (protons) in a strong magnetic field. The MRI machine uses gradient fields to create spatial differences, enabling precise imaging. The core components of MRI include a large magnet, gradient coils for imaging, and a radio transmitter and receiver to generate and capture signals. A computer processes these signals to create images.
• When placed in a magnetic field, hydrogen nuclei align and respond to radio waves, emitting a "spin echo" at a specific frequency (the "Larmor frequency"). The spin echo signal, containing spatial information, is processed using a "Fourier Transform" to create detailed images.

When MRI May Be Challenging

• Brain MRI requires the patient to lie within a narrow space for 10 to 30 minutes, which can be difficult for those with claustrophobia or severe anxiety. Communication with radiographers is limited during scanning due to the noise and enclosed environment.
• Open MRI scanners are available for those with severe claustrophobia, though access is often limited. The goal is to choose the shortest protocol that captures the critical clinical information needed for diagnosis.

Considerations for MRI Safety

• Metal outside the brain and eye is generally not an absolute contraindication for MRI. Magnetic deflection forces are minimal compared to normal physiological forces. Cardiac valves, inferior vena cava filters, biliary and vascular stents, IUDs, and metallic prostheses are typically safe unless positional stability is uncertain. For more detailed information, consult MRI Policies - Safety and Contraindications or MRI Safety.com.

MRI with Gadolinium

• Gadolinium is used in contrast-enhanced MRI to shorten T1 relaxation times, enhancing images. It is especially useful for detecting neoplastic or inflammatory lesions, abscesses, and meningeal disease, though not often used in acute stroke unless there is diagnostic uncertainty.

Understanding MRI Scan Sequences

• T1 Relaxation Time: The time for 63% of longitudinal magnetisation recovery. T1 is short in fat and long in water. Contrast agents like gadolinium shorten T1.
• T2 Relaxation Time: The time for 63% of transverse magnetisation loss. Liquids like water have a long T2, while larger molecules have shorter T2 times.
• T2* (T2 Star): Based on T2 decay with additional dephasing due to magnetic field inhomogeneities, relevant in gradient echo imaging.

T1-Weighted Imaging

• Parameters: Short TR/Short TE.
• Appearance:
  • Dark: CSF, oedema, tumour, infection, infarction, flow voids, calcification.
  • Bright: Fat, subacute haemorrhage, melanin, protein-rich fluid, slow-flowing blood, gadolinium. White matter is brighter than grey matter.
• T1 is ideal for anatomical details; acute stroke appears hypointense.

T2-Weighted Imaging

• Appearance:
  • Dark: Calcification, blood products, protein-rich fluid, flow void.
  • Bright: CSF, oedema, tumour, infarct, infection, subdural collections, subacute haemorrhage. Grey matter appears brighter than white matter.
• Good for identifying pathology, especially oedema and infarcts, which appear hyperintense after a few days.

Other Key Sequences

• T2* (Gradient Echo - GRE): Detects iron (e.g., haemosiderin) and haemorrhage; useful for identifying microbleeds.
• FLAIR (Fluid Attenuated Inversion Recovery): T2 sequence with suppressed free water signal, making CSF dark and highlighting periventricular oedema and lesions.
• Diffusion-Weighted Imaging (DWI): Detects cytotoxic oedema, valuable for early ischaemic stroke detection. Acute stroke appears bright on DWI and dark on ADC.
• Susceptibility-Weighted Imaging (SWI): Extremely sensitive to paramagnetic substances like blood and iron; useful for identifying microbleeds and distinguishing between haemorrhage and calcification.

Imaging Patterns in Stroke and Other Conditions

Imaging Patterns in Haemorrhagic Stroke