MRI Sequences and Parameters in Multiple Procedures
Introduction
MRI sequences are the setting of pulse sequences that give a particular image appearance. The contrast differentiation is very necessary for MRI images to differentiate normal anatomy from pathology. There are many different MRI sequences that make effective contrast differentiation between the tissues, which includes-
PD-Weighted Imaging
It is used to differentiate anatomical structures based on their proton density.
Proton density imaging is commonly used for brain imaging. The proton density (PD) weighted image is related to the number of protons (H nuclei) per volume.
Therefore Tissues with the higher density of protons produce the strongest signals and appear brighter on the image.
PD clearly distinguishes between gray and white matter, joint fluid, CSF muscle, etc. For PD images, Long TR/Short TE is used to minimize Ti and T2 effects.
T1-Weighted Imaging -
It is the basic pulse sequence. It demonstrates differences in the T1 relaxation times of tissues.
The contrast is determined by the difference in T1 relaxation times between fat and water. T1 weighted images are used to evaluate normal anatomy.
In tissue, fat has a shorter TI relaxation time than water. When an RF pulse of 90 degrees is given, fat will realign quicker than water in longitudinal magnetization. When RF pulses are repeated, fat will give high signals than water.
Therefore Tissues with high-fat content (e.g., white matter) appear bright and Tissues with high water content filled with water (e.g. CSF) appear dark. For T1-weighted images, a short TR and short TE sequence are used.
T1-weighted imaging can also be performed while injecting MRI contrast because Gadolinium shortens the T1 relaxation time of the tissue.
Gadolinium appears very bright on T1-weighted images. MRI contrast-enhanced images are useful in tumors, abscesses, inflammation, etc.
T2-Weighted MRI Image
T2-Weighted Imaging It demonstrates differences in the T2 relaxation times of tissues.
The contrast is determined by the difference in T2 relaxation times of various tissues.
Clinically T2 weighted images are used to evaluate pathology because most tissues are involved in a pathologic process and have a higher water content than normal.
Tissues with high water content filled with water (e.g. CSF) appear white. For T2-weighted images, a long TR and long TE sequence are used.
Fluid Attenuated Inversion Recovery (Flair) -
FLAIR is used to suppress the CSF signal so that pathology adjacent to the CSF is seen more clearly. It can be used in brain imaging to suppress cerebrospinal fluid (CSF) effects.
FLAIR is an inversion recovery sequence the Inversion recovery pulse sequence begins with a 180° pulse.
The 180° pulse inverts the magnetization vector in"Z. When the inverting pulse is removed, the magnetization vector begins to relax back and starts growing in the +Z direction.
Then after a time T1 a 90°excitation pulse is applied; hence the longitudinal magnetization flips into the x-y plane and the FID is produced and the signal of the CSF is nulled. FLAIR sequences are used to evaluate-
Infarction
Head injuries.
Subarachnoid hemorrhage
Multiple sclerosis, etc.
Diffusion Weighted Imaging (DWI) -
DWI is used to detect an acute stroke and other various pathology. It detects the random movements of water protons.
When water movement is restricted DWI gives a bright signal. The DW] sequences are spin-echo sequences, with 90- and 180-degree pulses.
The diffusion gradients are turned on before and after the 180-degree pulse. In general, highly cellular tissue or those with cellular swelling exhibits lower diffusion coefficients.
STIR (Short T1 Inversion Recovery) -
It is used to null the signal from fat. In MRI, fat has a high signal due to short relaxation times.
Therefore, tumors with surrounding fat are more difficult to detect on MRI. For STIR, an inversion recovery sequence with a short inversion time (TI) of 130-150 ms at 1.5 T is used for fat suppression.
1. BRAIN MRI (The Neuro Masterclass)
Protocol: T1 MPRAGE (3D), T2 FLAIR, DWI/ADC, SWI, MRS, Perfusion (DSC).
· T1 MPRAGE (3D): Gray matter looks grey, White matter looks white. Fat (scalp) is super bright. CSF is dark black. Bones are black. Advanced use: After contrast, tumors, meninges, and active MS plaques enhance (become white).
· T2 FLAIR: CSF is suppressed to dark. Periventricular white matter lesions (like demyelination or small vessel disease) shine bright white. Edema around tumors looks bright.
· DWI / ADC: Acute infarcts (stroke) are very bright on DWI and dark on ADC (restricted diffusion). Abscesses are bright on DWI. Epidermoid cysts are bright on DWI. Trick: If it is bright on DWI but also bright on ADC, it is "T2 shine-through" (not a real stroke).
· SWI (Susceptibility): Micro-hemorrhages, calcifications, and iron deposits look very dark/blooming. This is crucial for trauma, Parkinson's, and amyloid angiopathy.
· MRS (Spectroscopy): Tissue chemistry: NAA (Neuronal marker - low in stroke/tumor), Cho (Cell membrane - high in tumors), Lac (Lactate - peaks in ischemia/necrosis).
2. SPINE MRI (Cervical / Thoracic / Lumbar)
Protocol: Sag T1, Sag T2 STIR (Fat Sat), Axial T2 GRE, Axial T1.
· Sagittal T1: Vertebral bone marrow is bright (if yellow/fatty). Discs are intermediate grey. CSF is dark. Cord is intermediate. Advanced: If marrow is dark (replaces fat), suspect metastasis or multiple myeloma.
· Sagittal T2 STIR: Fat is completely nulled (dark). CSF is very bright (myelographic effect). Bone marrow edema (from fracture, infection, or Modic type 1 changes) becomes aggressively bright white. Disc bulge shows as dark extrusion.
· Axial T2 GRE: Ligamentum Flavum is dark black. Nerve roots are surrounded by bright CSF. Disc herniation looks like a dark focal protrusion compressing the bright CSF sac.
· Post-contrast T1 FS: Nerve roots enhance if inflamed (neuritis). Vertebral endplate enhancement means infection (spondylodiscitis).
3. CARDIAC MRI (The Functional King)
Protocol: Cine SSFP (Bright blood), T2 Dark-blood (HASTE), LGE (Late Gadolinium Enhancement - PSIR), T1/T2 Mapping.
· Cine SSFP (Bright blood): Blood pool is brilliantly white. Myocardium (heart muscle) is dark grey. Fat is bright. Function: You measure wall thickening and EF here.
· T2 Dark-blood (STIR): Blood is suppressed to dark to see the walls. Myocardial edema (acute myocarditis, acute MI) appears bright white on the dark background.
· LGE (Phase Sensitive IR): This is the gold standard. Normal myocardium is nulled to dark grey. Scar tissue / Fibrosis (from old MI or HCM) appears bright white. Thrombi inside the chamber appear dark (no perfusion).
· T1/T2 Mapping: Increased T1 means amyloidosis or diffuse fibrosis. Increased T2 means active inflammation (myocarditis). Advanced: ECV (Extracellular Volume) > 30% means diffuse pathology.
4. MRCP (Magnetic Resonance Cholangiopancreatography)
Protocol: 2D Thick-slab HASTE (Single-shot T2), 3D T2 SPACE (Isotropic), Respiratory-triggered.
· Heavy T2 weighting: Fluids are the only thing that are BRIGHT WHITE. This includes bile in the ducts, pancreatic juice, and gallbladder content.
· Everything else is suppressed to BLACK: Liver parenchyma, pancreas, fat, and blood vessels are completely dark.
· Advanced: If a duct is suddenly cut off with a dark filling defect, it's a stone (dark inside bright fluid). If there is a stricture with upstream dilated bright ducts, suspect malignancy (like pancreatic head tumor).
5. PELVIS MRI (Prostate / Uterus / Rectum)
Protocol: T2 TSE (High-res small FOV), DWI (B-50, 400, 1000), T1 VIBE DIXON (Dynamic contrast), ADC.
· T2 High-Res (Axial & Coronal):
· Prostate: Peripheral zone (PZ) is normally bright white. Central gland is dark grey. If PZ turns dark (loss of signal), suspect cancer (PIRADS 4/5).
· Uterus: Endometrium is bright white. Junctional zone (inner myometrium) is very dark. Outer myometrium is grey. Adenomyosis = thickened dark junctional zone.
· Bone marrow (pelvic bones) is bright (fat). Muscles are grey.
· DWI (B-1000) & ADC: Prostate cancer, endometrial cancer, and rectal tumors show restricted diffusion (Bright on DWI, Dark on ADC).
· T1 DIXON (In/Out phase): Detects microscopic fat in adrenal glands or bone marrow lesions. Hemorrhage (endometrioma) remains bright on T1 even without contrast.
6. WRIST MRI (Small joint precision)
Protocol: Coronal PD Fatsat (PROSET), Coronal T1, Axial T2 GRE (3D), Axial T1 + Contrast.*
· Coronal PD Fatsat: Fat is suppressed (dark). TFCC (Triangular Fibrocartilage) is a dark black triangular band. Scapholunate ligament is dark. Articular cartilage is intermediate grey. Bone marrow is dark (fat suppressed). Tendons (ECU, EPL) are dark black.
· Advanced: A TFCC tear = bright fluid (white) tracking into the normally dark ligament.
· Axial T2 GRE:* Median nerve is intermediate grey, surrounded by bright fluid in the carpal tunnel. If the nerve is swollen and dark, it is Carpal Tunnel Syndrome.
· Post-contrast T1: Synovitis (inflammation around the joints) enhances to white. Erosions in gout/RA show as dark bony defects.
7. KNEE MRI (Orthopedic Bread & Butter)
Protocol: Sagittal PD Fatsat, Sagittal T1, Coronal T2 Fatsat, Axial T2 Fatsat, 3D DESS (Cartilage).
· Sagittal PD Fatsat:
· Meniscus: Normally a black triangular wedge. A tear = a white line/cleavage jetting into that black triangle.
· ACL (Anterior Cruciate Ligament): Normally a dark, taut band. Torn ACL = bright edema (white) obscuring the ligament, with a wavy contour.
· PCL: Very dark and curved.
· Coronal T2 Fatsat:
· MCL (Medial Collateral Ligament): Dark band. Tear = white edema around it.
· Bone marrow edema (from a pivot-shift injury or occult fracture) = geographical bright white patches.
· Axial T2 Fatsat:
· Patellar cartilage is grey. Chondromalacia = bright white clefts in the cartilage.
· Hoffa's fat pad is suppressed (dark). If it is bright, it is impingement.
· 3D DESS (Double Echo Steady State): Articular cartilage is extremely bright (like water). Used to measure its thickness in millimeters.
8. SHOULDER MRI (The Rotator Cuff Mastery)
Protocol: Oblique Coronal T1, Oblique Coronal T2 FS, Oblique Sagittal T2 FS, Axial T2 FS, ABER (Abduction External Rotation) view.
· Oblique Coronal T2 FS:
· Supraspinatus tendon inserts into the humerus as a dark black band. A tear = bright white fluid (subacromial bursa fluid) tracking right through the dark tendon.
· Subacromial bursa is dark normally; bursitis = bright white fluid above the tendon.
· Oblique Sagittal T2 FS:
This is the "rotator cuff outlet" view. Supraspinatus muscle belly looks grey. Atrophy = bright white streaks (fatty infiltration) inside the grey muscle. Infraspinatus is below.
· Axial T2 FS:
· Labrum:
Dark triangular ring around the glenoid. SLAP tear or Bankart tear = bright fluid clefts between the dark labrum and the bone.
· Biceps tendon (long head) sits in the bicipital groove. Tendonitis = bright fluid around a thickened dark tendon.
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