Technique Axis¶

The Technique axis identifies the MRI pulse sequence family used for acquisition. It answers the question: "What pulse sequence was used to acquire this image?"

Overview¶

MRI techniques are grouped into four physics-based families:

Family	Physics	Key Characteristic	Examples
SE	Spin Echo	180° refocusing pulse	TSE, HASTE, SPACE
GRE	Gradient Echo	Gradient refocusing only	FLASH, MPRAGE, FIESTA
EPI	Echo Planar	Ultra-fast k-space traversal	Single/multi-shot EPI
MIXED	Hybrid	Combines physics or application-specific	DWI-EPI, BOLD, ASL, GRASE

Detection Strategy¶

NILS uses a three-tier detection approach (first match wins):

Tier 1: Exclusive Flag (95% confidence)¶

A single unified_flag that definitively identifies the technique:

MPRAGE:
  detection:
    exclusive: is_mprage  # If True → MPRAGE

Tier 2: Keywords Match (85% confidence)¶

Text pattern matching in series description:

MPRAGE:
  keywords:
    - "mprage"
    - "bravo"
    - "3d tfe"
    - "ir-fspgr"

Tier 3: Combination (75% confidence)¶

Multiple flags that must ALL be True (AND logic):

MPRAGE:
  detection:
    combination:  # All must be True
      - has_ir
      - has_gre
      - is_3d

Priority-Based Detection¶

Techniques are checked in priority order - more specific techniques before generic ones:

Specific → Generic

MPRAGE → checked before → GRE
RESOLVE → checked before → DWI-EPI → checked before → EPI
SPACE → checked before → TSE → checked before → SE

This ensures that MPRAGE (a specific IR-prepared 3D GRE) is detected rather than falling through to generic GRE.

SE Family (Spin Echo)¶

Uses 180° RF refocusing pulse. Produces "true" T2 contrast without susceptibility artifacts.

MDME (Multi-Dynamic Multi-Echo)¶

Vendor names: SyMRI, MAGiC

Physics: 2D TSE with multiple echoes at multiple TR/saturation times.

Detection: - Exclusive flag: is_mdme - Keywords: mdme, symri, magic

Purpose: Acquisition for synthetic MRI - generates T1, T2, PD maps and synthetic contrasts.

SPACE (3D-TSE)¶

Vendor names: Siemens: SPACE, GE: CUBE, Philips: VISTA

Physics: 3D volumetric TSE with variable flip angle refocusing (reduces SAR).

Detection: - Exclusive flag: is_space - Keywords: space, cube, vista, 3d tse - Combination: is_tse + is_3d

Purpose: Isotropic high-resolution 3D imaging.

HASTE (Single-Shot TSE)¶

Vendor names: Siemens: HASTE, GE: SSFSE, Philips: SSH-TSE

Physics: Acquires entire k-space in one TR (single-shot).

Detection: - Exclusive flag: is_haste - Keywords: haste, ssfse, ss-tse

Characteristics: - Very fast acquisition - Motion-robust - Has T2 blurring from long echo train

TIRM (IR-TSE)¶

Physics: Inversion recovery preparation + TSE readout.

Detection: - Keywords: tirm, stir, ir-tse - Combination: has_ir + is_tse

Variants: - STIR: Short TI for fat suppression - TIRM: Siemens IR-TSE variant

FLAIR vs IR-TSE

FLAIR is a modifier, not a technique. FLAIR uses IR-TSE physics but is captured in the Modifier axis. The technique would be TSE or IR-TSE.

MESE (Multi-Echo SE)¶

Physics: Multiple spin echoes at different TEs from single excitation.

Detection: - Exclusive flag: is_me_se - Keywords: mese, me-se, dual echo se - Combination: is_multi_echo + has_se

Purpose: T2 mapping, proton density estimation.

Note: Different from TSE - MESE has separate echoes; TSE has an echo train.

RESTORE (VFA-TSE)¶

Vendor names: Siemens: RESTORE, GE: FRFSE, Philips: DRIVE

Physics: Variable flip angle within echo train + driven equilibrium.

Detection: Keywords only (restore, frfse, drive)

Purpose: Optimized T2 contrast with improved SNR.

TSE (Turbo Spin Echo)¶

Vendor names: Siemens: TSE, GE: FSE, Philips: TSE

Physics: Echo train of refocusing pulses - multiple echoes per TR.

Detection: - Exclusive flag: is_tse - Keywords: tse, fse, turbo spin echo, rare

Clinical role: The workhorse of clinical MRI. Used for T1w, T2w, PD, FLAIR.

SE (Basic Spin Echo)¶

Physics: Simple 90°-180° pulse pair, single echo.

Detection: - Exclusive flag: has_se (fallback) - Keywords: spin-echo

Note: Fallback for SE family when no specific variant matches.

GRE Family (Gradient Echo)¶

Uses gradient reversal only (no 180° pulse). Faster than SE but susceptible to field inhomogeneities.

Quantitative/Special Techniques¶

QALAS (3D-QALAS)¶

Vendor: GE

Physics: Interleaved IR-GRE for multiparametric mapping.

Detection: - Exclusive flag: is_qalas - Keywords: qalas, 3d qalas

Purpose: T1, T2, PD quantification.

MP2RAGE¶

Physics: Two-inversion MPRAGE - acquires at two TI times.

Detection: - Keywords: mp2rage - Combination: is_multi_ti + has_gre + has_ir + is_3d

Purpose: Bias-reduced T1 mapping, uniform T1w images.

MEMPRAGE (Multi-Echo MPRAGE)¶

Physics: MPRAGE with multiple GRE echoes.

Detection: - Keywords: memprage, me-mprage - Combination: is_multi_echo + has_ir + has_gre + is_3d

Purpose: T2* mapping alongside structural T1w.

MPRAGE¶

Vendor names: Siemens: MPRAGE, GE: BRAVO/IR-SPGR, Philips: 3D-TFE

Physics: IR-prepared 3D spoiled GRE.

Detection: - Exclusive flag: is_mprage - Keywords: mprage, bravo, 3d tfe - Combination: has_ir + has_gre + is_3d

Clinical role: Standard 3D T1-weighted structural sequence.

MRA/Flow Techniques¶

TOF-MRA (Time-of-Flight)¶

Physics: Inflow enhancement - fresh spins appear bright.

Detection: - Exclusive flag: is_tof - Keywords: tof, time of flight

Purpose: Bright-blood angiography without contrast.

PC-MRA (Phase Contrast)¶

Physics: Bipolar gradients encode velocity.

Detection: - Exclusive flag: is_pc - Keywords: pc-mra, phase contrast, velocity

Purpose: Flow quantification, 4D flow imaging.

Multi-Echo GRE¶

MEDIC (Combined ME-GRE)¶

Vendor names: Siemens: MEDIC, GE: MERGE, Philips: mFFE

Physics: Fused multi-echo GRE.

Detection: Keywords: medic, merge, mffe

Implied base: T2*w (85% confidence)

MEGRE (Multi-Echo GRE)¶

Physics: Multiple gradient echoes at different TEs.

Detection: - Keywords: megre, me-gre, multi echo gre - Combination: is_multi_echo + has_gre

Purpose: T2* mapping, QSM, Dixon fat/water separation.

Implied base: T2*w (80% confidence)

SSFP Variants¶

CISS¶

Vendor names: Siemens: CISS, GE: FIESTA-C, Philips: 3D-DRIVE

Physics: Phase-cycled balanced SSFP.

Detection: - Exclusive flag: is_ciss - Keywords: ciss, fiesta-c

Purpose: High-SNR imaging of cisterns, cranial nerves.

FIESTA (bSSFP)¶

Vendor names: Siemens: TrueFISP, GE: FIESTA, Philips: bFFE

Physics: Fully balanced gradients - high SNR, mixed T2/T1 contrast.

Detection: - Exclusive flag: is_ssfp - Keywords: bssfp, truefisp, fiesta, balanced ffe - Combination: has_steady_state + has_gre

DESS (Dual-Echo SSFP)¶

Physics: Combines FID and echo signals.

Detection: Keywords: dess, mensa

Purpose: Musculoskeletal/cartilage imaging.

PSIF (Generic SSFP)¶

Physics: Reversed echo SSFP (T2-like contrast).

Detection: - Keywords: ssfp, psif, t2-ffe - Combination: has_steady_state + has_gre

Volumetric GRE¶

VIBE (3D Volumetric Spoiled GRE)¶

Vendor names: Siemens: VIBE, GE: LAVA, Philips: THRIVE

Physics: 3D spoiled GRE for volumetric T1 imaging.

Detection: - Keywords: vibe, lava, thrive - Combination: has_spoiled + is_3d + has_gre

VFA-GRE (Variable Flip Angle)¶

Physics: Multiple flip angles for T1 mapping.

Detection: - Keywords: vfa, despot - Combination: is_multi_fa + has_gre + is_3d

Fast/Basic GRE¶

TurboFLASH (Fast Spoiled GRE)¶

Vendor names: Siemens: TurboFLASH, GE: FSPGR, Philips: TFE

Detection: - Exclusive flag: is_tfl - Keywords: turboflash, fspgr, tfe

Purpose: Rapid T1w for dynamic imaging.

FLASH (Spoiled GRE)¶

Vendor names: Siemens: FLASH, GE: SPGR, Philips: T1-FFE

Physics: RF/gradient spoiling for pure T1 contrast.

Detection: - Exclusive flag: is_flash - Keywords: flash, spgr, t1-ffe - Combination: has_spoiled + has_gre

GRE (Generic)¶

Detection: - Exclusive flag: has_gre (fallback) - Keywords: gre, gradient-echo, ffe

Note: Fallback for GRE family.

EPI Family (Echo Planar Imaging)¶

Ultra-fast k-space traversal. Backbone for functional and diffusion imaging.

RESOLVE (Multi-Shot EPI)¶

Vendor names: Siemens: RESOLVE, GE: MUSE

Physics: Splits k-space over several shots - reduces distortion.

Detection: - Exclusive flag: is_epi_diff_resolve - Keywords: resolve, muse, multishot epi - Combination: has_segmented_kspace + has_epi

EPI (Generic)¶

Detection: - Exclusive flag: has_epi (fallback) - Keywords: epi, echo planar

MIXED Family (Hybrid Physics)¶

Combines SE+GRE, SE+EPI physics, or application-specific sequences.

Diffusion¶

DWI-EPI¶

Physics: SE-EPI with diffusion-sensitizing gradients.

Detection: - Exclusive flag: is_dwi - Keywords: dwi, dti, diffusion, hardi

Implied base: DWI (95% confidence)

DWI-STEAM¶

Physics: Stimulated echo diffusion.

Detection: Keywords: steam, dwi-steam

Functional/Perfusion¶

BOLD (BOLD-EPI)¶

Physics: GRE-EPI sensitive to deoxyhemoglobin.

Detection: - Exclusive flag: is_bold - Keywords: bold, fmri, functional, resting state

Note: No tissue contrast - functional imaging.

ASL (Arterial Spin Labeling)¶

Physics: RF labeling for non-contrast perfusion.

Detection: - Exclusive flag: is_asl - Keywords: asl, pcasl, pasl, arterial spin

Implied base: PWI

Perfusion-EPI¶

Physics: DSC (T2*-based) or DCE (T1-based) perfusion.

Detection: - Exclusive flag: is_perfusion - Keywords: perfusion, dsc, dce, pwi

Hybrid Physics¶

GRASE¶

Physics: SE refocusing with GRE readouts.

Detection: - Keywords: grase, turbogse, tgse - Combination: has_se + has_gre

Characteristics: Faster than pure TSE, less T2* than EPI.

SE-EPI¶

Physics: 180° SE preparation + EPI readout.

Detection: - Keywords: se-epi, spin echo epi - Combination: has_se + has_epi

Benefit: Less T2* artifact than GRE-EPI.

GRE-EPI¶

Physics: GRE preparation + EPI readout.

Detection: - Keywords: gre-epi, epi-gre - Combination: has_gre + has_epi

Note: Backbone for BOLD and DSC perfusion.

MRF (MR Fingerprinting)¶

Physics: Pseudo-randomized parameters + dictionary matching.

Detection: - Exclusive flag: is_mrf - Keywords: mrf, fingerprinting

Purpose: Quantitative T1/T2 mapping.

Important Notes¶

SWI is NOT a Technique¶

SWI (Susceptibility-Weighted Imaging) is a provenance/processing method, not an acquisition technique. The actual acquisition is GRE or EPI. SWI processing is captured in provenance="SWIRecon".

Radial/Spiral are Modifiers¶

Radial and Spiral are trajectory modifiers, not techniques: - Radial GRE = GRE technique + Radial modifier - Spiral EPI = EPI technique + Spiral modifier

These are captured in the Modifier axis.

Confidence Levels¶

Detection Method	Confidence	Rationale
Exclusive flag	95%	Definitive single flag
Keywords match	85%	Text-based identification
Combination (AND)	75%	Multiple flags required
Family fallback	60%	Generic family detection

Conflict Detection¶

NILS checks for physics family conflicts:

GRE technique detected but SE flags present → conflict
SE technique detected but GRE keywords in text → conflict

Conflicts trigger manual review. MIXED family techniques skip conflict checks (they inherently combine physics).

Technique-to-Base Inference¶

Some techniques strongly imply a base contrast:

Technique	Implied Base	Confidence
MPRAGE	T1w	95%
DWI-EPI	DWI	95%
TOF-MRA	T1w	90%
ME-GRE	T2*w	85%
ASL-EPI	PWI	95%
BOLD-EPI	None	-

BOLD-EPI has no tissue contrast inference (functional imaging).

YAML Configuration¶

Techniques are configured in backend/src/classification/detection_yaml/technique-detection.yaml:

techniques:
  MPRAGE:
    name: "MPRAGE"
    family: "GRE"
    description: "IR-prepped 3D T1w"
    keywords:
      - "mprage"
      - "bravo"
    detection:
      exclusive: is_mprage
      combination:
        - has_ir
        - has_gre
        - is_3d

rules:
  priority_order:
    - "MS-EPI"      # Specific first
    - "DWI-EPI"
    - "MPRAGE"
    # ... more specific ...
    - "GRE"         # Generic last
    - "EPI"