Diffusion-weighted magnetic resonance imaging

Whole-body DWI in treatment response assessment

The capabilities of diffusion-weighted MRI are clearly demonstrated when monitoring systemic oncological processes. A classic example is a patient with metastatic breast cancer receiving chemotherapy and bisphosphonates.

• Pre-treatment findings: In inverted 3D maximum intensity projection (MIP) images, widespread metastatic bone involvement is clearly visualized. These lesions appear as pronounced restricted diffusion areas.
• Post-treatment dynamics: With effective therapy, the prevalence of the disease decreases, and the areas of restricted diffusion shrink.
• ADC map analysis: For an objective assessment, color-coded apparent diffusion coefficient maps (ADC maps) are used. A positive response to ongoing therapy is manifested by an increase in ADC values in the affected zone (e.g., in the pelvic bones), which is accompanied by a color change on the generated map.

Magnetic resonance spectroscopy (MRS): Basic principles

While basic MRI shows anatomy, magnetic resonance spectroscopy provides a glimpse into the biochemical composition of tissues.

The method is based on measuring the peak frequency shift (measured in parts per million, ppm) relative to a given standard (most commonly water). This shift is unique for different substances, allowing precise identification of a specific molecule in the studied area.

At the same time, the area under the curve of the obtained peak directly indicates the concentration of the molecule relative to the standard. The main advantage of MRS is that it makes it possible to simultaneously detect multiple endogenous metabolites and assess their relative concentrations without a biopsy.

Proton MRS (1H-MRS) in neuro-oncology

Proton spectroscopy is based on the evaluation of hydrogen nuclei (each containing a single proton). This technique has been actively used since the 1980s, primarily to study the metabolism of central nervous system tumors.

Several key metabolites are distinguished in the spectrum:

1. N-acetylaspartate (NAA): Normally present predominantly in healthy neurons. If the graph shows a decrease or complete loss of the NAA peak, this indicates neuronal death (a typical picture for strokes or the growth of intracranial tumors).
2. Choline-containing molecules (Cho): Play a key role in the lipid biosynthesis pathway. Since malignant tumors are characterized by rapid growth and contain a significantly higher proportion of lipids compared to normal tissue, the choline peak acts as a dominant feature in the spectra of most neoplasms.
3. Creatine (Cr) and phosphocreatine (PCr): Are also visualized as separate peaks and serve as important reference points during interpretation.

Specific metabolites and spectroscopy on other nuclei

In certain rare types of cancer, specific metabolites themselves can stimulate tumorigenesis. For example, succinate (succinate) is an intermediate product of the tricarboxylic acid cycle. Its measurement using MRS is of high clinical significance for the diagnosis and treatment monitoring of patients with mutations in the succinate dehydrogenase enzyme.

In addition to hydrogen, MRS allows the assessment of other nuclei: carbon-13 (13C), fluorine-19 (19F), and phosphorus-31 (31P).

Phosphorus MRS (31P-MRS) is optimal for studying tissue energy status and cell membrane metabolism. It is capable of detecting the cell's basic energy unit — adenosine triphosphate (ATP). The tumor pattern in phosphorus MRS is characterized by altered membrane metabolism: high levels of phosphomonoesters (PME) are detected in the tissues.

Clinical applications of spectroscopy

To date, magnetic resonance spectroscopy has been integrated into several key clinical tasks in oncology and diagnostic radiology:

• Primary non-invasive diagnosis of neoplasms.
• Continuous monitoring of tumor response to ongoing treatment.
• Determination of the malignancy grade of the detected tumor (tumor grading).
• Precise delineation of hidden tumor margins for accurate radiotherapy planning.
• Assessment of local recurrence after surgical treatment or irradiation.

Class and exam questions

What else to study on this topic

• Fundamentals of MRI physics: Formation of inverted 3D maximum intensity projection (MIP) images
• Interpretation of ADC maps in non-oncological pathology
• The tricarboxylic acid cycle and the role of succinate dehydrogenase in oncogenesis
• Radiotherapy planning using MR spectroscopy data
• Differential diagnosis of local recurrence and radiation necrosis using MRS

Other topics in the "Diagnostic Radiology in Oncology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.