Image Formation Basics
Magnetic resonance occurs when tissues are exposed to radiofrequency pulses. Excited nuclei generate a signal that is spatially localized by a gradient field along three axes (slice, frequency, phase). The collected data fills k-space and is converted into an image via Fourier transformation.
Key timing parameters are repetition time (TR) between pulses and echo time (TE) before readout. There are two basic approaches:
- Spin Echo (SE): uses a 180-degree pulse. Provides clear T1-, T2-, and PD-weighted images. Modifications FSE and TSE allow for faster scanning.
- Gradient Echo (GE): operates without a 180-degree pulse, applying a gradient field instead. Significantly shortens TR (saves time), but is prone to signal loss due to magnetic field distortions.
Basic Modes: T1 and T2
Image contrast is traditionally evaluated relative to the brain's gray matter.
- T1-weighted images: white matter is brighter than gray matter. Fat appears hyperintense (bright), and CSF is hypointense (dark). A bright signal on T1 is also produced by melanin, protein, subacute hemorrhage (up to 28 days), and contrast (gadolinium).
- T2-Weighted Images: white matter is darker than gray matter, and CSF is bright. Pathological lesions most often appear bright due to increased water content (edema).
- FLAIR: a modification of inversion-recovery (IR) with CSF suppression. Ideal for detecting infarcts and demyelination. A bright signal in the sulci on FLAIR may indicate pus, subarachnoid hemorrhage, or artifacts from oxygen therapy.
- Fat Saturation (FS): a fat suppression mode. Helps identify lipomas or confirm blood within a vessel wall, but frequently produces artifacts at the skull base due to field inhomogeneity.
Diffusion and Perfusion Evaluation
For more advanced diagnostics, ultrafast echo-planar imaging (EPI) and other methods are used:
- DWI (Diffusion-Weighted MRI): evaluates the motion of water molecules (dependent on b-value). Acute infarcts, abscesses, and cellular tumors restrict diffusion. To confirm true restricted diffusion, the ADC map (apparent diffusion coefficient map) is analyzed to eliminate the T2 shine-through effect.
- DTI (Diffusion Tensor Imaging): studies diffusion in multiple directions to map white matter tracts.
- Perfusion: measures blood flow parameters (CBV, CBF, MTT). The DSC technique records signal drop during the passage of a contrast bolus, DCE assesses capillary permeability (important for gliomas), and ASL magnetically labels arterial blood without requiring contrast.
Spectroscopy (MRS) and fMRI
These techniques study tissue function and metabolism:
- MR Spectroscopy (MRS): non-invasively analyzes chemical composition. The N-acetylaspartate (NAA) peak indicates neuronal viability, choline indicates cell proliferation, lactate indicates anaerobic metabolism, and lipids appear with membrane damage. Sensitive to bone artifacts.
- Functional MRI (fMRI): maps cortical areas (motor, speech) prior to neurosurgery. Based on the BOLD effect: active neurons induce a local influx of blood, changing the ratio of oxygenated to deoxygenated hemoglobin, which is detected by the scanner.
Briefly: answers to topical questions
How do acute and chronic stages of hemorrhage appear on T1- and T2-weighted images?
The acute stage of hemorrhage is characterized by decreased signal intensity, while specific characteristics of the chronic stage on T1- and T2-weighted images are not described in the sources.
| Hemorrhage Stage | T1-Weighted Images | T2-Weighted Images |
|---|---|---|
| Acute | Hypointense rim (rim) | Hypointense signal (due to deoxyhemoglobin); hypointense rim |
| Chronic | Not described (hematomas may calcify or organize into cysts) | Not described |
What is the b-factor (b-value) in diffusion-weighted MRI and how does its value affect image contrast?
The sources do not provide a physical definition of the b-factor, but indicate that signal intensity on diffusion-weighted images directly depends on its value.
- High b-factor (b-value) — using such values (e.g., b1400) makes it possible to detect areas of restricted diffusion (restricted diffusion), which are characteristic of tumor tissue.
On T1 scans, everything is "anatomical": white matter is bright, and fluid (CSF) is dark. On T2, it is a "water kingdom": water (CSF and pathological edema) glows bright white.
Class and exam questions
Why is the FLAIR mode needed if there is a standard T2?
FLAIR works like T2, but purposefully suppresses the bright signal from normal CSF. This allows clear visualization of pathological lesions (demyelination, infarction) that would otherwise blend with ventricular fluid.
How does acute ischemic stroke appear on DWI scans?
Due to cytotoxic edema, the diffusion of water molecules in the infarction zone is sharply restricted. On DWI, this appears as a bright hyperintense lesion, and on ADC maps, as dark.
What is the SWI mode used for?
SWI has high magnetic susceptibility sensitivity. This mode is indispensable for detecting microbleeds (deoxyhemoglobin, hemosiderin) and calcifications in brain tissue.
What else to study on this topic
- Mathematics of MRI: Properties of k-Space and Fourier Transformation
- MRI Artifacts: Influence of Paramagnets and Oxygen Therapy
- Physiological Basis of the BOLD Effect and Neurovascular Coupling
- MR Spectroscopy with Short Echo Time (Short TE): Myo-Inositol and Glutamate
- Principles of Non-Contrast ASL Perfusion (Arterial Spin Labeling)
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