Arterial Spin Labeling (ASL)
The Arterial Spin Labeling (ASL) technique differs fundamentally from standard contrast-enhanced studies. It uses magnetic labeling of the patient's own blood hydrogen.
- Advantages: It does not require the administration of gadolinium-based agents, making it ideal for examining children and patients with contraindications to contrast agents.
- Measured parameters: This is the only perfusion method that directly measures regional cerebral blood flow (rCBF). ASL-derived rCBF values correlate well with cerebral blood volume (rCBV) obtained using contrast-enhanced techniques (e.g., DSC PWI).
- Limitations: Because labeled hydrogen freely crosses the blood-brain barrier, ASL cannot measure intravascular blood volume, unlike DSC.
Metabolites and Patterns of MR Spectroscopy (MRS)
MR spectroscopy analyzes the chemical composition of a selected tissue volume. It is particularly useful for differentiating tumors from non-neoplastic processes, although less accurate in determining specific histological types.
Typical pattern of a malignant glioma (compared to the healthy contralateral hemisphere):
- Decrease in N-acetylaspartate (NAA) and creatine. NAA is a neuron-specific marker; its decline indicates the death or displacement of normal neurons.
- Increase in choline (Cho). Reflects accelerated synthesis and breakdown of cell membranes in areas of high mitotic activity.
- Appearance of lactate. The end product of non-oxidative glycolysis. Exacerbated by hypoxia in rapidly growing tumors, which stimulates angiogenesis.
- Presence of mobile lipids. Indicates membrane breakdown and tissue necrosis.
An important parameter is the echo time (Time of Echo, TE). A short TE (20–40 ms) shows more metabolites (myo-inositol, glutamate) but produces baseline artifacts. An intermediate TE (135–144 ms) provides a stable baseline and accurate quantification of NAA and choline, while inverting the lactate peak.
Microstructure Assessment: DWI and DTI
Diffusion-weighted imaging (DWI) evaluates water molecule motion, which depends on cellular density and the tumor matrix. The apparent diffusion coefficient (ADC) is calculated, with values inversely correlating with the Ki-67 proliferation index. Marked reduction in diffusivity is characteristic of specific tumors, such as lymphoma.
Diffusion tensor imaging (DTI) evaluates the directionality of diffusion.
- Normally, white matter tracts have high fractional anisotropy (FA).
- Tumor infiltration disrupts myelin sheaths, decreasing anisotropy.
- DTI data are used for tractography—the generation of color-coded connectivity maps for preoperative planning.
Functional MRI and PET
To minimize surgical deficits, fMRI is used. The method is based on the BOLD effect (blood oxygenation level-dependent signal) while the patient performs tasks. This indirectly localizes eloquent (speech, motor) areas. However, activation within a tumor may be false due to angiogenesis or susceptibility artifacts.
PET with standard glucose ([18F]-FDG) is limited in neuro-oncology due to the physiologically high glucose uptake of the healthy brain (the tumor blends with the background). Therefore, alternative tracers are used:
- Amino acid analogues ([11C]-MET, [18F]-FET);
- Radiolabeled choline (membrane assessment);
- 68Ga-DOTATOC (highly specific for meningiomas).
Briefly: answers to topical questions
Which specific brain tumors are characterized by a marked decrease in the apparent diffusion coefficient (ADC) on DWI?
A marked decrease in the apparent diffusion coefficient (ADC) / diffusivity on DWI is described in sources for lymphoma and medulloblastoma; lower ADC values also help identify malignant gliomas.
- Lymphoma — a marked decrease in diffusivity can characterize this specific neoplasm.
- Medulloblastoma — demonstrates restricted diffusion on DWI/ADC, which is a sign of high cellularity.
- Malignant gliomas — lower ADC values, without true restricted diffusion, help identify them.
What cerebral hemodynamics parameters are evaluated in contrast-enhanced DSC perfusion?
In contrast-enhanced DSC perfusion / dynamic susceptibility contrast MR perfusion, sources evaluate relative cerebral blood volume (rCBV).
- rCBV makes it possible to visualize areas of elevated blood volume within a malignant glioma.
- rCBV is used for the differential diagnosis of active tumefactive demyelinating lesions and gliomas: in Grade IV gliomas, a marked increase in rCBV is noted, whereas in tumefactive demyelinating lesions, rCBV is normal or slightly elevated.
What types (patterns) of white matter tract involvement by tumors exist according to DTI tractography?
According to DTI tractography, the following types of white matter tract involvement by tumors are distinguished:
- Infiltration — tumor cells destroy myelin sheaths, leading to a loss of anisotropy.
- Peripheral displacement — separation of fibers without tumor invasion (observed in ependymoma).
- Fiber deviation — tracts may be displaced by the tumor in a specific direction (e.g., anteriorly in astrocytoma).
Which metabolites are additionally visualized on MR spectroscopy when using a short echo time (TE 20–40 ms)?
When using a short echo time (TE 20–40 ms) on MR spectroscopy, the following metabolites are additionally visualized:
- Myo-inositol (myo-inositol)
- Glutamate (glutamate)
- Glutamine (glutamine)
- Lipids (Lipids)
Visualization of these compounds improves tumor characterization; however, the technique is prone to baseline distortion and artifactual peaks.
The malignant glioma pattern on MRS can be remembered using the 'scissors' rule: the choline peak shoots upward (active division), while the peak of the healthy neuronal marker NAA drops downward, and pathological lactate and lipid peaks appear on the baseline.
Class and exam questions
Why is standard FDG-PET rarely used for brain tumors?
Healthy brain tissue has a physiologically very high baseline glucose metabolism, making tumor tissue contrast poorly against healthy structures.
What is the advantage of intermediate echo time (TE) in MR spectroscopy?
It provides a flat baseline, allows for more accurate evaluation of NAA and choline, and inverts the lactate peak, facilitating its identification on the spectrogram.
What does fractional anisotropy (FA) show on DTI maps?
It quantitatively reflects the tendency of water to diffuse in a single direction. It is high in healthy myelinated tracts and decreases when they are infiltrated and destroyed by a tumor.
What else to study on this topic
- Limitations and Artifacts of fMRI (Impact of Angiogenesis and Blood Breakdown Products on the BOLD Signal)
- Multivoxel Spectroscopy (CSI) vs. Single-Voxel: Advantages of 3D Arrays
- Specifics of Using Amino Acid Radiopharmaceuticals in PET
- Comparison of rCBF parameters in ASL technique and rCBV in DSC technique
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