Veterinary MR Calculators & Reference
Small-animal ready-reckoners for dog & cat MRI: formulae, worked examples and pre-computed lookup tables.
Contrast dosing
Gadolinium dose
Standard extracellular gadolinium-based contrast agent (GBCA) dose is 0.1 mmol/kg body weight. The injected volume depends on the agent’s molar concentration.
Volume (mL) = Dose (mmol) ÷ concentration (mmol/mL)
Most agents (gadoterate / gadoteridol / gadobenate) are 0.5 mmol/mL → 0.2 mL/kg. Gadobutrol is concentrated at 1.0 mmol/mL → 0.1 mL/kg.
Lookup table (0.1 mmol/kg)
| Weight (kg) | Dose (mmol) | mL @ 0.5 mmol/mL | mL @ 1.0 mmol/mL |
|---|---|---|---|
| 2 | 0.2 | 0.4 | 0.2 |
| 4 | 0.4 | 0.8 | 0.4 |
| 6 | 0.6 | 1.2 | 0.6 |
| 10 | 1.0 | 2.0 | 1.0 |
| 15 | 1.5 | 3.0 | 1.5 |
| 20 | 2.0 | 4.0 | 2.0 |
| 30 | 3.0 | 6.0 | 3.0 |
| 40 | 4.0 | 8.0 | 4.0 |
| 50 | 5.0 | 10.0 | 5.0 |
Cats receive the same weight-based dose (small cats typically 2–6 kg). Reduce or avoid GBCA in renal disease. Typically 0.1 mmol/kg; verify against local protocol & product label.
Contrast dosing
Contrast for MR angiography & portosystemic shunt studies
Contrast-enhanced MRA and portosystemic (liver) shunt studies use the same GBCA dose class as routine imaging (≈0.1 mmol/kg); use the Gadolinium dose table for volumes. The critical variable is timing, not dose.
Success = correct bolus timing relative to vessel of interest
Key timing points:
- Arterial-phase MRA: image during first pass of the bolus; a rapid power-injected or hand bolus followed by saline flush, with bolus-tracking or a test bolus/fluoro-trigger where available.
- Portosystemic shunt: dynamic multi-phase acquisition (arterial → portal venous → delayed) to catch the anomalous vessel; portal venous phase is usually most informative.
- A short, compact bolus and reliable saline flush matter more than the absolute millimolar amount.
Typically same 0.1 mmol/kg dose class with phase-optimised timing; verify against local protocol & product label.
Workflow
Anaesthesia-adjacent quick refs & scan-time planning
General anaesthesia is required for diagnostic small-animal MRI (motion-free, immobile patient). Plan total table/anaesthesia time from the sequence list, not just the “scan” time.
Rough sequence-time planning guide
| Study element | Typical planning time |
|---|---|
| Positioning & coil set-up / localisers | 5–10 min |
| Single anatomical sequence | 3–8 min each |
| Typical brain / spine protocol (multi-sequence) | 25–45 min |
| Add post-contrast series | +5–15 min |
| Low-field magnet (longer averaging) | often 1.5–2× longer |
Plan generously and build in repeat-sequence contingency. Follow your practice anaesthetic protocol and direct all drug dosing to the anaesthetist. Typical planning ranges only; verify against local protocol.
Safety concept
SAR / RF heating
Specific Absorption Rate (SAR) quantifies radiofrequency (RF) energy deposited in tissue, expressed in watts per kilogram (W/kg). RF energy from the transmit coil is absorbed and turned into heat.
SAR ↑ with B₀², flip angle, echo-train length & RF duty cycle
Why it matters in small-animal MRI
- Small patients: a cat or toy-breed dog has little tissue mass to distribute and dissipate heat; localised heating can be relatively greater than in a large patient, and thermoregulation under anaesthesia is impaired.
- Implants & foreign bodies: conductive materials (microchips, leads, some plates) can concentrate RF energy and heat locally; the transmit coil drives this, not the receive coil.
- Field strength: RF heating rises roughly with the square of B₀, so higher-field magnets deposit more energy for the same sequence.
- Mitigation: lower flip angles, fewer/less-dense RF pulses, longer TR, and monitoring anaesthetised patient temperature.
No numeric per-patient calculation is required; the scanner reports and limits SAR. Understand the principle and monitor the anaesthetised patient. Verify against local protocol & scanner safety limits.
Sequence parameter
TI-to-null (STIR / FLAIR)
To null a tissue in an inversion-recovery sequence, set the inversion time (TI) to the zero-crossing of that tissue’s longitudinal recovery:
STIR nulls fat (short T1); FLAIR nulls fluid / CSF (long T1). T1 lengthens with field strength, so the required TI changes with B₀.
Approximate T1 & resulting TI (ms) by field strength
| Tissue | 0.2 T | 0.5 T | 1.0 T | 1.5 T |
|---|---|---|---|---|
| Fat: T1 | ~140 | ~180 | ~230 | ~260 |
| Fat: TI (STIR) | ~97 | ~124 | ~159 | ~179 |
| CSF / fluid: T1 | ~2400 | ~2900 | ~3800 | ~4300 |
| CSF: TI (FLAIR) | ~1656 | ~2001 | ~2622 | ~2967 |
STIR fat suppression is relatively field-independent and robust at low field; it relies on T1 nulling rather than chemical-shift, making it the workhorse for vet magnets where spectral fat-sat is unreliable. T1 values are approximate and tissue-/vendor-dependent. Typical values; verify against local protocol.
Image quality
SNR & field-strength ready-reckoner
Signal-to-noise ratio scales (qualitatively) with field strength and with voxel volume:
Relative SNR & practical implication
| Field | Relative SNR (approx.) | Practical implication |
|---|---|---|
| 0.2 T | ~0.13× | Thicker slices / larger voxels, more averages, longer scans |
| 0.5 T | ~0.33× | Compromise resolution; favour STIR over spectral fat-sat |
| 1.0 T | ~0.67× | Good balance for most small-animal work |
| 1.5 T | 1.0× (reference) | Thinner slices / higher resolution achievable in less time |
To recover SNR lost at low field you can increase voxel size, add signal averages (SNR ∝ √N, so 4× averages → 2× SNR at 4× the time), or lengthen the scan, all with anaesthetic-time consequences.
Relative SNR figures scale linearly with B₀ and are indicative only; real SNR also depends on coil, sequence and tissue. Verify against local protocol.
Image quality
Voxel volume & scan-time trade-off
Voxel volume is the product of the in-plane pixel dimensions and slice thickness:
In-plane = 160 ÷ 256 = 0.625 mm per pixel → voxel = 0.625 × 0.625 × 3 = 1.17 mm³.
Coarsen matrix to 128×128 → 1.25 × 1.25 × 3 = 4.69 mm³ (4× the volume → ~4× the SNR, at the cost of resolution).
Trade-off cheat sheet
| Change | Effect on SNR | Effect on scan time / detail |
|---|---|---|
| Double voxel volume | ~doubles SNR | Lower spatial detail |
| Halve slice thickness | ~halves SNR | Finer detail, more slices |
| Double averages (NEX) | ×√2 (~1.4) | Doubles scan time |
| Finer matrix | Lower SNR | Higher resolution, longer scan |
Note: doubling voxel volume approximately doubles SNR, the primary lever for low-field vet magnets. Typical relationships; verify against local protocol.
Handy conversions
Unit conversions for vet MRI
mL = mmol ÷ concentration (mmol/mL)
Body weight: pounds ↔ kilograms
| Pounds (lb) | Kilograms (kg) |
|---|---|
| 2 | 0.9 |
| 5 | 2.3 |
| 10 | 4.5 |
| 15 | 6.8 |
| 22 | 10.0 |
| 33 | 15.0 |
| 44 | 20.0 |
| 66 | 29.9 |
| 88 | 39.9 |
| 110 | 49.9 |
Gadolinium: mmol ↔ mL for the two concentrations
| Dose (mmol) | mL @ 0.5 mmol/mL | mL @ 1.0 mmol/mL |
|---|---|---|
| 0.25 | 0.5 | 0.25 |
| 0.5 | 1.0 | 0.5 |
| 1.0 | 2.0 | 1.0 |
| 2.0 | 4.0 | 2.0 |
| 3.0 | 6.0 | 3.0 |
| 5.0 | 10.0 | 5.0 |
Typical conversions using 1 kg = 2.2046 lb; verify weights on a calibrated scale and doses against the product label.
Educational reference only. This page is not a substitute for the product label, the scanner’s safety documentation, or veterinary and anaesthetic clinical judgement. All contrast and anaesthetic decisions rest with the attending clinician.
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Educational reference for imaging professionals. It does not replace local policy, manufacturer labelling or clinical judgement.