MRI Examination Techniques: Core Methods and Functional Extensions

Modern MRI examination strategy integrates a foundational anatomical protocol with targeted functional and physiologic add‑ons tailored to the clinical question. Conventional (“routine”) MRI encompasses a structured set of pulse sequences designed to exploit differential proton relaxation characteristics across tissues. In contrast to computed tomography, which acquires axial source data for later planar reformatting, MRI can directly acquire high‑resolution axial, sagittal, coronal, and oblique planes without geometric interpolation loss. A typical baseline protocol for neuro, spine, musculoskeletal, abdominal, or pelvic evaluation includes at minimum T1‑weighted, T2‑weighted, and sometimes proton density or fluid‑attenuated sequences; the exact combination reflects organ system and suspected pathology. Image contrast arises from pulse repetition time (TR), echo time (TE), and—in gradient echo or inversion-based approaches—flip angle and inversion time (TI). Classical spin echo T1 weighting is produced with short TR and short TE, while T2 weighting emerges from long TR and long TE; proton density weighting uses a long TR with a relatively short TE to minimize both T1 and T2 contrast, emphasizing subtle internal structure. Multi-slice, multi-echo spin echo and rapid refocusing variants (fast or turbo spin echo) improve efficiency by collecting multiple lines of k-space per TR. Inversion recovery techniques (e.g., STIR for fat suppression via fat’s specific T1 null point, or FLAIR for cerebrospinal fluid suppression) selectively attenuate tissues whose longitudinal magnetization crosses zero at prescribed TI values. Gradient echo sequences (2D or 3D) exploit variable flip angles and shorter TR to accelerate acquisition or enhance susceptibility effects, supporting angiography, cartilage mapping, and dynamic contrast imaging.

Contrast-enhanced MRI typically employs gadolinium-based paramagnetic chelates, which shorten T1 relaxation and, to a lesser extent, T2 in regions where they distribute extracellularly or intravascularly, thereby increasing signal on T1‑weighted sequences. Unlike iodinated CT contrast where radiodensity correlates with iodine concentration, MRI signal response plateaus and depends on baseline relaxation environment, field strength, pulse sequence design, and local concentration. Pre-contrast acquisition establishes baseline lesion characteristics (intrinsic T1/T2, diffusion metrics, susceptibility behavior); dynamic or delayed post-contrast phases then reveal enhancement kinetics distinguishing breakdown of the blood–brain barrier, tumor neovascularity, fibrosis, or inflammatory hyperemia. In many indications a post-contrast 3D gradient echo T1 sequence provides isotropic data for multiplanar reformation and leptomeningeal, vascular, or cranial nerve evaluation. Fat suppression (frequency-selective, Dixon-based, or STIR) after contrast accentuates subtle enhancement, particularly in marrow, breast, orbital, and musculoskeletal imaging.

Magnetic Resonance Angiography (MRA) harnesses flow-related phenomena to depict blood vessels without ionizing radiation. Time-of-flight (TOF) MRA leverages inflow enhancement: unsaturated spins flowing into an imaging slab appear hyperintense against partially saturated stationary background, facilitating intracranial arterial evaluation. Phase-contrast (PC) MRA encodes velocity information into phase shifts, enabling both anatomic visualization and quantification of volumetric flow or regurgitant fractions—valuable in cerebrospinal fluid dynamics, congenital heart disease, and venous assessment. Contrast-enhanced MRA uses dynamic T1 shortening from gadolinium bolus passage captured during peak arterial phase, producing high signal vascular lumens across extensive fields with reduced saturation artifacts; view-sharing or time-resolved approaches (e.g., TWIST, TRICKS) add temporal separation of arterial and venous phases for shunt or malformation analysis. Emerging non-contrast techniques such as arterial spin labeling and balanced steady-state free precession variants expand vascular and perfusion interrogation in patients with contraindications to contrast agents.

Magnetic Resonance Spectroscopy (MRS) extends MRI’s spatially localized assessment into biochemical profiling by isolating frequency-specific resonance peaks of metabolites—most commonly in the brain. Whereas conventional MRI depicts morphology and relaxation-based contrast, single-voxel or multi-voxel (chemical shift imaging) spectroscopy characterizes relative concentrations of compounds such as N-acetylaspartate (neuronal integrity), choline (membrane turnover), creatine (energy reserve reference), lactate (anaerobic metabolism), myo-inositol (glial content), and lipid/macromolecular peaks (necrosis or demyelination). Clinical applications include differentiating neoplasm from radiation necrosis, grading tumor aggressiveness adjunctively, evaluating metabolic disorders, and assessing epilepsy foci. Acquisition demands magnetic field homogeneity optimization (shimming), water and lipid suppression, and appropriate echo time selection balancing signal-to-noise with spectral resolution. Interpretation is inherently semi-quantitative unless absolute quantitation techniques and calibration references are employed.

These core methods integrate seamlessly with further functional and quantitative platforms—diffusion-weighted imaging for acute ischemia and tumor cellularity assessment, diffusion tensor imaging for white matter tractography, perfusion (dynamic susceptibility or dynamic contrast enhancement) for microvascular hemodynamics, susceptibility-weighted imaging for paramagnetic or calcific detection, and emerging quantitative mapping sequences providing standardized biomarkers for longitudinal follow-up. Strategic tailoring of protocol components yields a high-value examination aligned with the specific clinical indication while optimizing total scan time, patient comfort, and artifact resilience.

Disclaimer: Educational summary; protocol specifics should follow institutional standards and published professional guidelines.