23.3 MRI Examination Protocols, Patient Preparation, Gadolinium Contrast & Sectional MRI Anatomy
Key Takeaways
- An MRI protocol is built from a planning localiser plus a combination of T1-weighted, T2-weighted, fluid-attenuated and fat-suppressed sequences in at least two orthogonal planes, with post-contrast T1 sequences where indicated.
- On T1-weighted images fat is bright and fluid is dark, on T2-weighted images fluid is bright, and on fluid-attenuated inversion recovery cerebrospinal fluid is suppressed so that periventricular lesions become conspicuous.
- Gadolinium chelates shorten T1 and therefore brighten enhancing tissue on T1-weighted sequences; they carry a risk of nephrogenic systemic fibrosis in severe renal impairment, so renal function is checked before administration.
- Screening before entry to Zone IV must identify cardiac pacemakers and implanted electronic devices, ferromagnetic aneurysm clips, cochlear implants, metallic ocular foreign bodies, and pregnancy, and screening applies to accompanying relatives and to any staff entering the room.
- Absolute contraindications relate to devices and foreign bodies rather than to the examination itself, and every implant must be verified as MR safe, MR conditional or MR unsafe before scanning.
23.3 MRI Examination Protocols, Patient Preparation, Gadolinium Contrast & Sectional MRI Anatomy
Methodology and Magnetic Resonance Imaging Anatomy carries 6 items and asks the examinee to explain the MRI examination in terms of protocol, patient preparation, use of contrast agent, and limitations of examination, and to illustrate the basic magnetic resonance imaging anatomy. Section 23.1 gave the physics and Section 23.2 the safety zones and artifacts; this section is the working examination.
1. How a Protocol Is Assembled
Every MRI protocol has the same skeleton:
- Localiser (scout) in three planes — a fast, low-resolution acquisition used only for planning.
- A T1-weighted sequence for anatomy.
- A T2-weighted sequence for pathology, because most pathology increases water content.
- A fluid-suppressed or fat-suppressed sequence appropriate to the region.
- At least two orthogonal planes, so that a lesion is confirmed in more than one dimension.
- Post-contrast T1-weighted sequences, usually with fat suppression, when enhancement is required.
Reading the weighting
| T1-weighted | T2-weighted | FLAIR | STIR | |
|---|---|---|---|---|
| Parameters | Short TR, short TE | Long TR, long TE | Long TR, long TE, long TI | Short TI, inversion recovery |
| Fluid (CSF, urine, bile) | Dark | Bright | Suppressed (dark) | Bright |
| Fat | Bright | Intermediate to bright | Bright | Suppressed (dark) |
| Best for | Anatomy, fat, subacute blood, post-contrast enhancement | Pathology, oedema, fluid collections | Periventricular and cortical brain lesions | Marrow oedema, occult fracture, soft-tissue oedema |
The single most useful heuristic: T2 is the "water" sequence — pathology is usually bright on T2. T1 is the "fat and anatomy" sequence, and it is the sequence on which gadolinium enhancement is seen.
Additional sequences to recognise:
- Gradient echo (GRE) and susceptibility-weighted imaging — sensitive to blood products, calcium and metal, which "bloom" as dark signal loss.
- Diffusion-weighted imaging (DWI) with an apparent diffusion coefficient (ADC) map — restricted diffusion is bright on DWI and dark on ADC, the signature of acute infarction, of abscess, and of some hypercellular tumours.
- MR angiography — time-of-flight or contrast-enhanced, for vessels.
- MR cholangiopancreatography (MRCP) — heavily T2-weighted, so static fluid in the biliary and pancreatic ducts is bright while everything else is dark; requires fasting.
- MR spectroscopy — metabolite quantification.
- Functional MRI — blood-oxygen-level-dependent contrast for cortical mapping.
Representative protocols
| Study | Core sequences |
|---|---|
| Brain (routine) | Sagittal T1; axial T2; axial FLAIR; axial DWI/ADC; axial GRE or SWI; coronal T2; post-contrast axial, coronal and sagittal T1 if indicated |
| Cervical or lumbar spine | Sagittal T1; sagittal T2; sagittal STIR; axial T2 through the levels of interest; post-contrast T1 with fat saturation for infection, tumour or post-operative assessment |
| Knee | Sagittal proton density and PD fat-saturated; coronal T1 and T2 fat-saturated; axial PD fat-saturated |
| Liver | Axial in-phase and opposed-phase T1; axial T2 fat-saturated; DWI; dynamic post-contrast T1 in arterial, portal venous and delayed phases |
| MRCP | Coronal and axial heavily T2-weighted thin sections plus a thick-slab radial acquisition; fast 4-6 hours |
| Pelvis / prostate | High-resolution T2 in three planes; DWI with high b value; dynamic contrast-enhanced T1 |
| Cardiac | Cine steady-state free precession; T2 for oedema; late gadolinium enhancement for scar or fibrosis |
2. Patient Preparation and Screening
The screening questionnaire — before the patient reaches Zone III
The following must be actively asked and documented, for the patient, for any accompanying relative or carer entering the room, and for staff:
| Category | Examples |
|---|---|
| Cardiac and neurostimulation devices | Pacemaker, implantable cardioverter-defibrillator, loop recorder, deep-brain stimulator, vagus nerve stimulator, spinal cord stimulator |
| Vascular implants | Aneurysm clips (ferromagnetic ones are the classic hazard), coils, stents, filters, mechanical heart valves |
| Ear and eye | Cochlear implant; metallic ocular foreign body — a history of metal grinding or a penetrating eye injury requires orbital radiographs before scanning |
| Infusion and drug delivery | Insulin pump, implanted infusion pump, medication patch containing metal foil |
| Orthopaedic and dental | Plates, screws, prostheses, external fixators, orthodontic appliances |
| Other | Shrapnel or bullet fragments, tissue expanders with metallic ports, IUD type, body piercings, tattoos with metallic pigment |
| Physiological | Pregnancy, breastfeeding, renal function, claustrophobia, ability to lie still, weight and girth against table limits |
Every device must be classified as MR Safe (no known hazard), MR Conditional (safe only under specified field strength, gradient and SAR conditions) or MR Unsafe (never enter Zone IV). "It's probably fine" is not a classification.
Immediately before the scan
- Change into a gown; remove all metal — jewellery, watches, hairpins, hearing aids, dentures with metal, removable prostheses, coins, keys, phones, credit and access cards.
- Explain the noise and provide ear protection — acoustic noise regularly exceeds 100 dB.
- Provide the emergency call bell and confirm the patient can operate it.
- Position for comfort; support the knees and back; use the appropriate radiofrequency coil for the region.
- Manage claustrophobia with explanation, prone positioning, feet-first entry, a mirror, a companion in the room (also screened), or prescribed anxiolysis.
- Confirm fasting where required — MRCP and hepatobiliary or bowel-contrast studies.
- Immobilise and use breath-hold or respiratory triggering for body work; motion is the dominant cause of MRI repeats.
3. Gadolinium Contrast
Mechanism. Gadolinium is strongly paramagnetic. Chelated to prevent free-ion toxicity, it shortens T1 relaxation time in tissue where it accumulates, so enhancing tissue becomes bright on T1-weighted images. It is not "seen" directly; it changes the signal of the tissue it reaches.
| Feature | Detail |
|---|---|
| Typical dose | About 0.1 mmol per kilogram of body weight, followed by a saline flush |
| Route | Intravenous, usually via a power injector for dynamic studies |
| Indications | Tumour detection and characterisation, infection and abscess, inflammation and demyelination activity, vascular imaging, post-operative assessment, cardiac scar imaging |
| Adverse reactions | Much less frequent than with iodinated contrast; nausea, headache, transient taste disturbance, injection-site coolness; anaphylactoid reactions are rare but possible, so emergency drugs and trained staff must be available |
| Nephrogenic systemic fibrosis (NSF) | A rare, serious fibrosing disorder associated with gadolinium exposure in severe renal impairment. Assess renal function before administration; use the lowest necessary dose of a lower-risk agent; avoid where contraindicated |
| Pregnancy | Gadolinium crosses the placenta; avoid unless the benefit clearly outweighs the risk |
| Breastfeeding | Very little is excreted in milk and less is absorbed; current guidance generally permits continued breastfeeding, but follow local policy |
| Gadolinium retention | Trace deposition in brain and bone has been documented; clinical significance is unestablished, but it reinforces using contrast only when indicated |
MRI is not free of risk simply because it uses no ionising radiation. The hazards are the static field (projectile effect), gradient fields (acoustic noise and peripheral nerve stimulation), radiofrequency energy (heating, measured as specific absorption rate, and burns from conductive loops), cryogens (quench and asphyxiation risk), and contrast.
4. Limitations of MRI
Be able to state these directly, because the TOS names limitations of examination as a competency.
| Limitation | Consequence |
|---|---|
| Long acquisition times | Motion sensitivity; poor tolerance in unwell, paediatric or claustrophobic patients; low throughput |
| Device and implant contraindications | Some patients cannot be scanned at all |
| Susceptibility artifact from metal | Large signal voids and distortion near prostheses; CT may be superior |
| Poor for cortical bone and calcification | Both are signal-void; CT is superior for fracture detail and calcium |
| Lung parenchyma | Low proton density and severe susceptibility effects; CT is the modality of choice |
| Acute haemorrhage detection in trauma | Non-contrast CT remains first-line for speed and access |
| Access and cost | Limited availability in many Philippine provincial settings; higher cost than CT or ultrasound |
| Claustrophobia and body habitus | Bore diameter and table weight limits |
| Monitoring difficulty | Only MR-conditional monitoring, infusion and anaesthetic equipment may enter Zone IV |
5. Basic Sectional MRI Anatomy
The technologist checks every series before releasing the patient, so the normal appearances must be recognisable.
Brain
- Axial at the level of the lateral ventricles: frontal horns, caudate heads, thalami, third ventricle, internal capsule with its anterior limb, genu and posterior limb, and the occipital horns.
- Axial at the midbrain: cerebral peduncles, the "Mickey Mouse" midbrain outline, quadrigeminal plate cistern, temporal lobes.
- Sagittal midline: corpus callosum with rostrum, genu, body and splenium; the pituitary in the sella; the brainstem with pons and medulla; cerebellar vermis; fourth ventricle; and the cerebral aqueduct.
- Coronal: hippocampi in the medial temporal lobes, lateral ventricles, and the relationship of the internal carotid arteries to the cavernous sinus.
- CSF check: dark on T1, bright on T2, suppressed on FLAIR. If CSF is bright on your "FLAIR", the sequence is mislabelled or the inversion time is wrong.
Spine
Sagittal images show the vertebral bodies with fatty marrow (bright on T1), the intervertebral discs (bright on T2 when normally hydrated), the CSF-filled thecal sac, the conus medullaris ending at about L1-L2, and the cauda equina below it. Axial images show the disc, the thecal sac, the exiting nerve roots in the neural foramina, and the facet joints.
Knee
Sagittal images show the anterior and posterior cruciate ligaments, the menisci as low-signal triangles, the patellar tendon and the articular cartilage. Coronal images show the collateral ligaments and the medial and lateral menisci. Normal ligaments and menisci are uniformly low signal; increased internal signal reaching an articular surface indicates a tear.
Abdomen and pelvis
Recognise the liver segments, portal and hepatic veins, gallbladder and biliary tree, pancreas with the splenic vein posterior to it, kidneys with corticomedullary differentiation on T1, adrenal glands, aorta and inferior vena cava, and in the pelvis the zonal anatomy of the uterus (bright endometrium, dark junctional zone, intermediate myometrium on T2) or of the prostate (bright peripheral zone on T2, from which most carcinoma arises).
A brain MRI series is reviewed before the patient is released. On one sequence the cerebrospinal fluid is dark while the subcutaneous fat is bright, and on another the cerebrospinal fluid is bright. Which sequences are these?
A patient scheduled for lumbar spine MRI reports that he works as a metal grinder and once had a metal fragment removed from his eye. What must be done before he enters Zone IV?
Why is a gadolinium-based contrast agent visible as bright signal on post-contrast T1-weighted images?
Which limitation makes computed tomography rather than MRI the first-line examination in acute major trauma?