Cardiac MRI (CMR)
Source: Short Notes Cardiology (SNC5) + live verify (27 ก.ค. 2026: T2 iron cutoffs, adenosine adequacy). Cardiac MRI (CMR)* = gold standard สำหรับ LV/RV volume & function, tissue characterization (edema/scar/iron), viability. Scan ~45 นาที. Companion: Stress Imaging · Myocarditis · Cardiac Sarcoidosis · Cardiac Mass — Tumors & Thrombus · Approach to Cardiomyopathies (Hub)
🧲 Patient Safety
- Quench = ปล่อยไอ liquid He เพื่อดับ field ฉุกเฉิน (ยาก/เสี่ยง/แพง)
- Hazardous implants (contraindicated): cochlear implant, neurostimulator, hydrocephalus shunt, metal ocular implant, pacing wires, metallic cerebral aneurysm clip (list เต็ม mrisafety.com)
- Safe: sternal wires, mechanical valves, annuloplasty rings, coronary stents, nonmetallic catheters, orthopedic/dental implants
- Claustrophobia: oral sedation หรือ large-bore scanner
- Nephrogenic systemic fibrosis (gadolinium): low risk ใน CKD, 2.5–5% ใน ESRD
- Contraindications เพิ่ม: aortic graft (Zenith stent), MRI-incompatible device, unknown-source cerebral aneurysm clip, abandoned/conventional lead, recent lead implant <6 สัปดาห์
💉 Pharmacologic Stress & Gadolinium
- Gadolinium dose: standard 1 mL/kg; perfusion half-dose 0.5 mL/kg; viability 1 mL/kg
- Gadolinium ไป interstitium, เข้า cell เฉพาะเมื่อ membrane abnormal (acute infarct/fibrosis); less sensitive ใน multivessel disease; จำกัดเมื่อ GFR <30
- Adenosine 140 mcg/kg/min × 3–4 นาที: ประเมิน function/perfusion/viability; SE = bronchospasm/chest tightness (A1 receptor); infarct-size post-ACS รอ 7 วัน (stable)
- Vasodilator adequacy: hemodynamic response หรือ splenic switch-off (spleen ↓ signal = ตอบสนอง adenosine พอ; HR rise เดียวไม่พอ)
- Dobutamine (max 40 mcg/kg/min ± atropine 0.3–0.6 mg): ใช้แทนถ้า adenosine C/I
- Only pharmacologic ที่ไป stress → rest (ต่างจากอื่น)
🧬 Sequences
| Sequence | ใช้ดู |
|---|---|
| Black blood T1 | anatomy |
| CINE-SSFP (bright blood) | function (global/regional) + anatomy; bright regurgitant jet = severe |
| Coronary (T2 fat-suppressed) | black pericardium, ostial/proximal lesions, lipomatous hypertrophy |
| Perfusion | ischemia (subendocardial/transmural) |
| Early enhancement (T1 FSE) | hyperemia ใน myocarditis (dark blood, white myocardium), abnormal ≥4 |
| Delayed enhancement (LGE) | scar (gray blood, white scar; transmural/subendocardial); false-positive ใน acute MI; white-black-white = core infarct/hemorrhage; thrombus = black |
| T2-weighted (black blood, gray myocardium) | edema (↑signal) |
| Long-TI (white blood) | thrombus (black) vs myocardium (gray) |
| Velocity-encoded | flow quantification |
| T2* | iron load |
🩸 T2* Iron Overload (verified 27 ก.ค. 2026)
| T2* | แปลผล |
|---|---|
| ≥20 ms | ปกติ (high NPV, ไม่มี clinically significant siderosis) |
| <20 ms | myocardial siderosis (clinically significant) |
| <10 ms | severe iron overload (เสี่ยง LV dysfunction/HF) |
🔎 Interpretation (signal intensity: hyper/iso/hypo)
- Stunning: perfusion ปกติ
- Hibernation: perfusion defect ตอน stress
- Infarct: scar (LGE)
- Calcium: black
Scar / ischemia extent: transmural scar >50% wall thickness, subendocardial <50%; “ischemic beyond scar” นับเฉพาะเมื่อ ไม่ใช่ (transmural ischemia + submural scar). Exam key: MVO (microvascular obstruction, หาย 4–6 สัปดาห์), RV infarction, myocardial edema, thrombus, pericarditis
🗂️ Scar Patterns by Layer
| Pattern | สาเหตุ |
|---|---|
| Mesocardial (midwall) | HCM, DCM, PHT |
| Patchy | sarcoid, amyloid, myocarditis |
| Transmural | infarction (พบบ่อยสุด), severe/chronic myocarditis, sarcoid |
| Subendocardial — vascular | infarction |
| Subendocardial — non-vascular | amyloid, hypereosinophilic syndrome, histiocytoid CM, transplant |
| Subepicardial | myocarditis (พบบ่อยสุด), sarcoid |
- HCM: RV-insertion scar / patchy infiltration
- Myocarditis: ↑T2 (edema) + ↑early gadolinium T1 (hyperemia) + LGE midwall (~40%); eosinophilic = subendocardial → ดู Myocarditis (Lake Louise 2018)
- Amyloid: ~80% มี scar pattern (subendocardial/diffuse); ICM: 100% scar
🧭 Indications & Reading Checklist
- Preferred: Rt-side valve eval, quantifying regurgitation, LV/RV volume & function
- Limitations: underestimate AS velocity ถ้า Vmax >4 m/s; eyeball regurgitation estimation = pitfall
- Reading order (ischemia study): aorta (R/O dissection) + PA size (R/O thrombus) → pericardial/pleural effusion, extracardiac mass, lung infiltrate → global/regional LV & RV function → wall thickness, LA/RA dilation, IAS thickness → LV/LA/RV/RA thrombus → perfusion defect location → scar pattern (LV/RV/atrium/MVO) → pericardial effusion/LGE
🚨 STRICT AVOIDANCE / RED FLAGS
- ห้าม/ระวัง MRI: cerebral aneurysm clip (unknown source), cochlear implant, neurostimulator, metal ocular implant, MRI-incompatible device, abandoned lead, lead implant <6 สัปดาห์
- Gadolinium: หลีกเลี่ยงเมื่อ GFR <30 (NSF risk 2.5–5% ใน ESRD)
- CMR underestimate AS velocity เมื่อ Vmax >4 m/s → ใช้ Doppler echo สำหรับ severity จริง
- Adenosine: bronchospasm/chest tightness (A1) — เตรียม aminophylline; ตรวจ splenic switch-off ยืนยัน adequate stress
🎯 High-Yield Recall
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Gold standard: LV/RV volume & EF, Rt-side valve, regurgitation quantification, tissue characterization
-
LGE (delayed enhancement) = scar (white); transmural >50% = ไม่ recover หลัง revascularization; MVO หาย 4–6 สัปดาห์
-
T2 <20 ms = iron overload, <10 ms = severe*
-
Scar pattern: subendocardial vascular = infarct; subepicardial/midwall = myocarditis; patchy = sarcoid/amyloid; RV insertion = HCM
-
Myocarditis: ↑T2 (edema) + early gad (hyperemia) + midwall LGE (Lake Louise 2018)
-
Adenosine 140 mcg/kg/min, adequacy = splenic switch-off; gadolinium C/I GFR <30
-
🔍 Verification status
✅ Searched & verified (27 ก.ค. 2026):
- T2 iron cutoffs*: <20 ms = clinically significant siderosis, <10 ms = severe, ≥20 ms normal (high NPV) — Anderson/Pennell, Circulation & PMC reviews — ✅ ตรง source
- Adenosine 140 mcg/kg/min standard dose + splenic switch-off เป็น marker of adequacy (HR rise ไม่พอ) — EHJ-CI, JCMR — ✅
- NSF risk 2.5–5% ใน ESRD, gadolinium จำกัด GFR <30 — สอดคล้อง ACR guidance
⚠️ From source (SNC5), stable knowledge — ไม่ได้ re-search รายค่า:
- Sequence-to-tissue mapping (SSFP/LGE/T2W/early enhancement), scar-pattern-by-layer table, transmural >50% viability rule, MVO 4–6 wk — standard CMR textbook (SCMR)
- Lake Louise myocarditis criteria อยู่/verified ใน Myocarditis