Medical Radiology - Lazar A.P. 2008

Diagnostic imaging methods
Magnetic resonance imaging

Magnetic Resonance Imaging (MRI) is the youngest radiological diagnostic method. The history of magnetic resonance began in 1946 when two American physicists, F. Bloch and E.M. Purcell, discovered a fundamental physical phenomenon: the magnetic resonance of atomic nuclei of certain elements in the periodic table. Based on the nuclear magnetic resonance (NMR) phenomenon, magnetic resonance spectroscopy (MRS) was developed, allowing the presence of various substances in a studied object to be assessed. For a long time, it was used primarily by physicists, chemists, and biologists. In 1971, American physicist P. Lauterbur proposed a method for the spatial localization of the MR signal, based on weak magnetic field gradients and back-projection image reconstruction techniques already utilized in computed tomography. Eight years later, the first whole-body magnetic resonance scanner appeared in clinical practice (Fig. 13).

The main modalities of magnetic resonance Diagnostics are magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS).

Magnetic resonance imaging utilizes very low energy from the very beginning of the electromagnetic spectrum. The energy used in MRI is 19 orders of magnitude lower than that used in X-ray and radionuclide imaging Methods. MR scanners can generate cross-sectional images of any part of the body without using ionizing radiation. Compared to Ultrasonography and X-ray Computed Tomography, this technique is more expensive, technically more complex, and theoretically harder to understand. Nevertheless, MRI has revolutionized diagnostic radiology.

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Fig. 13. Modern Siemens Medical Systems magnetic resonance scanners: A – closed-bore system (with an enclosed magnetic coil);

B – open system (with an open magnetic coil).

The core principle of magnetic resonance imaging is nuclear magnetic resonance (NMR)—the resonance of atomic nuclei placed in a constant magnetic field in response to a specific radiofrequency pulse, accompanied by their own electromagnetic emission. Most currently operating magnetic resonance scanners are tuned to the resonance of the simplest dipole nuclei with an odd mass number: the hydrogen Nucleus (Fig. 14).

The magnetic field is measured in teslas or gauss: 1 T = 10,000 G. Clinical practice employs magnetic resonance scanners with magnetic field strengths of 0.3, 0.5, 1.0, and 1.5 T, which significantly exceed the Earth's magnetic field (ranging from 0.3 G at the equator to 0.7 G at the poles). A constant magnetic field of 0.5 T or higher is generated by an electromagnet utilizing superconductivity. The stronger the magnetic field used in the device, the better the resulting image quality. The scanner's gradient coils constantly adjust, altering the magnetic field strength, frequency, and phase of the electromagnetic wave in the transverse (x, y) and longitudinal (z) planes, enabling detectors to pinpoint the exact Location emitting a radiofrequency pulse of a specific frequency and intensity. Each voxel (from volume and Cell) of the studied object corresponds to a pixel (from picture and cell) of the image on the computer monitor.

Fig. 14. Mechanism of nuclear magnetic resonance formation.

1 – hydrogen nuclei; 2 – magnet; 3 – radiofrequency electromagnetic radiation.

In addition to visualizing the topographical Structure of Organs and Tissues based on proton density across all planes (axial, sagittal, coronal), magnetic resonance imaging detects two relaxation times (T₁, T₂) and fluid flow velocity. Tissues with different physical properties exhibit distinct T₁ and T₂ values, which are obtained following exposure to a series of radiofrequency pulses at specific time intervals by configuring the scanner with preset T_R and T_E parameters, where T_R is the repetition time (time between successive radiofrequency pulses) and T_E is the echo time (time between pulse transmission and signal reception). T₁-weighted (spin-lattice or longitudinal) relaxation is characterized by short T_R and T_E values—approximately 20 ms and 600 ms, respectively—whereas T₂-weighted (spin-spin or transverse) relaxation features long T_R and T_E values—approximately 80 ms and 3000 ms, respectively.

Proton density-weighted images feature a long T_R and a short T_E. On T₂-weighted images, fluids and consequently inflamed areas produce an intense signal, allowing pathological processes to be visualized more clearly. T₁-weighted imaging is superior for assessing the Morphology of the area under investigation. On T₁-weighted images, adipose tissue, proteinaceous substances, and gadolinium (a paramagnetic contrast agent used in MRI) yield an intense signal and appear bright; Brain parenchyma, dense Internal Organs, Blood vessel walls, and Muscles appear less bright (intermediate); while Bone tissue, calcifications, and air appear dark. White and grey matter, as well as malignant and normal Tissues of the same type, yield different T₁ signals. The ability to detect pathological conditions across various organs and tissues underscores the high diagnostic value of MRI.

Non-contrast vascular visualization allows for two- and three-dimensional volumetric rendering of Blood Vessels (magnetic resonance angiography), while high-speed magnetic resonance scanners can distinguish between Arteries and Veins based on blood flow direction.

Current scientific research has revealed no harmful biological effects associated with MRI Procedures. Contraindications for magnetic resonance imaging include: 1) the presence of foreign metallic objects in the patient's body (metallic surgical implants, vascular clips, shrapnel) that may shift under METABOLISM/18.html">The Influence of a strong magnetic field; 2) the presence of a cardiac pacemaker, which may malfunction during the scan; 3) the first trimester of Pregnancy; and 4) Epilepsy. Additionally, prolonged confinement (ranging from several to tens of minutes) within a closed magnetic resonance scanner may induce claustrophobia in some patients.

Fig. 15. Principle of Ultrasound examination.

A – propagation of ultrasound across tissue interfaces; B – A-mode echography.

1 – ultrasound transducer emitting ultrasonic waves (shaded arrow) and receiving reflected ultrasonic waves (black arrow).

Magnetic resonance spectroscopy, which qualitatively and quantitatively determines The chemical composition of organs and tissues, relies on the phenomena of nuclear magnetic resonance and chemical shift. Chemical shift is based on the principle that The Nucleus of a specific chemical element absorbs energy at different frequencies across the electromagnetic spectrum depending on the surrounding molecular structure. MRS is most commonly performed on hydrogen (protons) and phosphorus nuclei. This examination helps differentiate between benign and malignant tumors and determine their histological type. Although MRS is a promising magnetic resonance technique, Technical Challenges in its execution and data interpretation, along with lengthy scan times, limit its routine clinical application.