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CT Calcium Scoring (Coronary Artery Calcium Scoring — CAC)

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1. Definition

"Coronary calcium is an indicator of the burden of atherosclerotic plaque, although there is no correlation of the amount of local coronary calcium with the physiologic or anatomic significance of an underlying coronary stenosis." — Goldman-Cecil Medicine

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2. Principle

Pathophysiological Basis

• Calcification in the coronary arteries occurs almost exclusively in the setting of coronary atherosclerosis (with the exception of advanced chronic kidney disease).
• As the amount of coronary calcium roughly correlates to the extent of atherosclerotic plaque, its detection and quantification allows risk stratification.
• Most patients with acute coronary syndrome (ACS) have coronary calcium, and their calcium burden is substantially greater than in age- and sex-matched controls without CAD.
• Importantly, coronary calcification is not related to plaque stability/instability and is only weakly related to the severity of luminal stenosis — i.e., a heavily calcified plaque may not be obstructive.

Physical Principle

• CT acquires images by passing a thin X-ray beam through the body at multiple angles. X-ray transmission measurements are digitized into pixels — each assigned a value in Hounsfield units (HU), referenced to water (0 HU).
• Calcium is highly attenuating and appears bright white on non-contrast CT, even without contrast enhancement.
• A coronary lesion is defined as calcium when CT density is ≥ 130 HU with an area of ≥ 1 mm².

Agatston Score Calculation

• The Agatston score corresponds to each coronary lesion's calcium area multiplied by the maximal CT attenuation value, then summed for the entire coronary tree.
• Less commonly, volume score (mm³) and calcium mass score (mg) are also used.

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3. Score Interpretation / Categories

• Scores are age, sex, and race dependent; they are normalized and reported as percentile scores against population databases.
• Overall 15-year mortality rates in asymptomatic patients range from 3 to 28% for CAC scores from 0 to ≥ 1000.
• A CAC score of 0 (zero) is associated with excellent prognosis; future cardiac events are highly unlikely.
• CAC adds prognostic information independently of standard risk factors (Framingham risk score).

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4. Equipment Required

CT Scanner

• Multi-detector CT (MDCT) scanner — the standard platform today
  • Minimum: 16-slice MDCT; optimal: 64-slice or higher
  • Modern scanners: 128–640 slice systems (dual-source CT, wide-detector CT)
• Electron Beam CT (EBCT) — the original scanner used for CAC scoring (now largely replaced by MDCT)
  • Advantages of EBCT: very fast acquisition, less cardiac motion artefact
  • Disadvantage: less widely available

ECG Synchronization

• ECG gating hardware — mandatory for cardiac motion suppression
  • Prospective ECG triggering is standard for CAC scoring (triggers acquisition at a fixed phase of the cardiac cycle, typically mid-diastole)
  • Reduces motion blur, minimises radiation dose

Other Required Equipment

• ECG electrodes and monitoring leads (3 or 4 lead setup)
• Cardiac calcium scoring post-processing software (e.g., Syngo.via [Siemens], AW workstation [GE], Extended Brilliance Workspace [Philips])
• Software automatically identifies lesions ≥ 130 HU and calculates per-vessel and total Agatston scores with color-coded overlays

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5. Indications

• Asymptomatic adults at intermediate cardiovascular risk (10–20% 10-year event risk) — most validated indication, can "upgrade" or "downgrade" risk category
• Asymptomatic patients at low risk with a family history of premature ischaemic heart disease
• Patients where statin therapy decision is uncertain after clinical risk assessment
• Not indicated in symptomatic patients (a negative CAC score does not exclude obstructive CAD in young or symptomatic patients)

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6. Contraindications / Limitations

• Symptomatic patients — a zero CAC score does not reliably exclude CAD (especially in young patients with non-calcified soft plaques)
• Patients with known CAD or prior revascularisation — rarely changes management
• Already low-risk or already high-risk patients — adds limited incremental value (a very high CAC may prompt stress testing)
• Pregnancy — ionising radiation contraindicated
• Advanced CKD — calcification may occur without atherosclerosis, reducing specificity
• Irregular heart rhythm (AF) — impairs ECG gating, though modern software can compensate

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7. Patient Preparation

1. No iodinated contrast is required — this is a plain (non-contrast) scan
2. Patient should avoid caffeine and smoking for several hours before the scan (may affect heart rate)
3. Ideally, heart rate should be below 65–70 bpm for adequate gating, though CAC is more forgiving than CTA
4. Beta-blockers may be given to lower heart rate if needed (more critical for CT angiography)
5. Explain the procedure; no special fasting required (unlike contrast studies)
6. Remove metallic objects from the chest area
7. Attach ECG electrodes to the chest

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8. Scan Protocol — Step by Step

Step 1: Scout / Topogram

• Acquire a low-dose AP scout image of the chest
• Used to define the scan range: from the carina (level of the tracheal bifurcation) down to the diaphragm, encompassing the entire heart

Step 2: ECG Electrode Placement and Monitoring

• Attach 3–4 lead ECG electrodes to the patient's chest
• Verify a clean ECG signal on the console
• Prospective ECG triggering: acquisition is triggered at ~70–75% of the R–R interval (mid-diastole), when the heart is most still

Step 3: Breath-Hold Instruction

• Instruct the patient to take a moderate breath-hold (not a forced deep inspiration) during acquisition
• Typically 10–15 seconds

Step 4: Acquisition Parameters

Note: Slice thickness of 2.5–3 mm is standardised for Agatston scoring; thinner slices (used in CT angiography) do not yield directly comparable Agatston scores.

Step 5: Radiation Dose

• Radiation dose is intentionally very low: approximately 1–2 mSv (comparable to a few months of background radiation)
• Prospective gating significantly reduces dose compared to retrospective spiral acquisition

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9. Image Reconstruction and Post-Processing

Step 1: Reconstruction

• Images are reconstructed at 2.5 mm slice thickness with a medium-smooth kernel
• Iterative reconstruction algorithms (e.g., SAFIRE, iDose, ASIR) reduce image noise — allow diagnostic quality even at low dose

Step 2: Transfer to Calcium Scoring Workstation

• DICOM images are sent to dedicated cardiac scoring software

Step 3: Automated Lesion Detection

• Software automatically identifies all foci ≥ 130 HU with area ≥ 1 mm²
• Highlights them with color-coded overlays by coronary territory (e.g., LM = cyan, LAD = green, RCA = red, LCx = blue)
• Assigns density weight factors (1–4) based on peak HU

Step 4: Manual Review and Editing

• The radiologist/technologist reviews each detected lesion
• False positives must be excluded: papillary muscle calcification, mitral/aortic valve calcifications, sternal wires, pericardial calcification — these are NOT coronary and must be deleted from the ROI
• False negatives from automated algorithms can occur in extensive calcification

Step 5: Per-Vessel Score Reporting

• Scores are reported per vessel (LM, LAD, LCx, RCA) and as a total
• Volume (mm³) is also calculated as a secondary metric
• Total score is compared to age/sex/race-matched percentile tables and the patient is placed in a risk percentile

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10. CT Images — Grades of CAC

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11. Reporting

1. Total Agatston score
2. Per-vessel Agatston scores (LM, LAD, LCx, RCA)
3. Volume score (total, mm³)
4. Age/sex/race-adjusted percentile
5. Risk category (absent / minimal / mild / moderate / severe)
6. Incidental findings on the scan (lung nodules, pleural effusion, aortic aneurysm, etc.) — important added value of every CAC scan
7. Clinical correlation and recommendation (e.g., discuss with clinician regarding statin initiation)

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12. Clinical Utility

• Primary prevention risk stratification: CAC is currently the strongest imaging-based predictor of future MI and cardiac death in asymptomatic patients
• Useful to reclassify intermediate-risk patients — a zero score may defer statin therapy; a high score may escalate it
• High scores may prompt stress testing even in otherwise low-symptom patients
• CAC can also be acquired as a by-product during hybrid SPECT/CT or PET/CT scans
• CAC scoring does not require contrast, making it suitable for patients with renal insufficiency or contrast allergy when coronary CTA would be contraindicated

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13. Radiation Dose Comparison

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Summary

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• Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Ch. 248 & 284
• Goldman-Cecil Medicine, International Ed., Ch. 44
• Grainger & Allison's Diagnostic Radiology, Ch. 15
• Fuster and Hurst's The Heart, 15th Ed., Ch. 21

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CT Perfusion Scanning (CTP) — In Detail

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1. Definition

"CT perfusion scans generate quantitative color maps that indicate various physiologic parameters such as cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) through quantitative analysis of rapidly acquired image sequences during intravenous contrast administration." — Schwartz's Principles of Surgery

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2. Principle

Physiological Basis

1. Vasodilation → increases CBV, maintains CBF
2. Blood cells spend longer in the tissue → MTT prolongs
3. If vessels are maximally dilated and CPP drops further → CBF falls
4. Oxygen extraction increases maximally, then cellular dysfunction begins
5. When CBF < 15–20 mL/100g/min → loss of electrical neuronal function
6. When CBF < 10 mL/100g/min → ATP failure → Na⁺/K⁺ pump failure → cytotoxic oedema → irreversible infarction

The Penumbra Model

Physical / Mathematical Principle

1. A bolus of iodinated contrast is injected IV
2. The contrast transiently increases CT attenuation (HU) as it passes through tissue
3. Repeated CT acquisitions are made rapidly over the same slices — generating a time-density curve (TDC) at every pixel
4. Two curves are generated: the arterial input function (AIF) from a large artery, and the tissue curve from brain parenchyma

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3. Key Parameters — Normal Values and Significance

• Core = CBF severely reduced AND CBV reduced (microvascular collapse)
• Penumbra = CBF moderately reduced BUT CBV preserved (autoregulation still working)
• Mismatch = Penumbra volume ≥ 20% larger than core → favourable for reperfusion therapy

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4. Exam Diagram

How to Draw in Exam — Step by Step

HU
↑        AIF (tall, sharp peak)
|       /\
|      /  \      Tissue curve (low, broad)
|    /    \   /‾‾‾\
|  /      \_/      \___
+________________________→ Time (sec)

Area under tissue curve = CBV
Peak height / steepness = related to CBF
MTT = CBV ÷ CBF

• CBF → dark hole on one side (reduced)
• CBV → smaller hole (core only reduced; penumbra preserved)
• MTT → large bright/prolonged area
• Tmax → largest area of delay (>6s = penumbra threshold)

         Benign oligaemia (CBF 20-50)
       ┌──────────────────────────┐
       │   Ischaemic penumbra     │
       │    ┌──────────────┐      │
       │    │  INFARCT     │      │
       │    │    CORE      │      │
       │    │ CBF < 10     │      │
       │    └──────────────┘      │
       │     CBF 10–20            │
       └──────────────────────────┘
              CBF 20–50

|████████████████████████| ← Penumbra volume
|████████|                ← Core volume
         ↑
    Mismatch ratio ≥ 1.8 → TREAT (thrombectomy/thrombolysis)

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5. CTP Perfusion Maps — Clinical Images

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6. Equipment Required

CT Scanner

• Multi-detector CT (MDCT) — minimum 64-slice; optimal 128-slice or higher
• Wide-detector CT (256–320 slice) — covers the entire brain in a single rotation without table movement; reduces radiation and motion artefact (preferred)
• Older systems required toggling (shuttling) the table back and forth to cover the whole brain
• Modern photon-counting CT — improved resolution and reduced dose

Contrast Injector

• Dual-head power injector — delivers contrast at precise rate (typically 4–6 mL/sec) followed by saline flush
• Required for tight, reproducible bolus

Monitoring

• ECG monitoring (not mandatory but useful if arrhythmia suspected)
• Pulse oximetry and IV access

Post-Processing Software

• Dedicated perfusion workstation software (e.g., Syngo.via [Siemens], IntelliSpace [Philips], Advantage Workstation [GE], RAPID [iSchemaView] — the most validated for stroke)
• Software performs:
  • Automated AIF selection
  • Deconvolution algorithms
  • Colour map generation
  • Core vs. penumbra segmentation
  • Mismatch ratio calculation

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7. Indications

Neurological (Primary — Brain CTP)

1. Acute ischaemic stroke — most important indication
   • Identify ischaemic core vs. salvageable penumbra
   • Guide decision for IV thrombolysis (tPA) and mechanical thrombectomy
   • Extended time window (6–24 hours) — patient selection by DAWN/DEFUSE-3 criteria
2. Transient ischaemic attack (TIA) — detect occult hypoperfusion
3. Vasospasm after subarachnoid haemorrhage — assess delayed cerebral ischaemia
4. Brain tumours — assess vascularity, grade, distinguish recurrence from radiation necrosis
5. Pre-surgical cerebrovascular reserve assessment

Non-Neurological Uses

• Acute pancreatitis — detect pancreatic necrosis earlier than conventional CT
• Oncology — assess tumour perfusion, angiogenesis, treatment response (lung, liver, pancreas)
• Pulmonary nodules — differentiate malignant from benign

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8. Patient Preparation

1. IV access — large-bore antecubital vein (18G or larger), right arm preferred (avoids left brachiocephalic vein artefact on CTA)
2. Assess renal function — eGFR > 30 mL/min/1.73m² preferred; in acute stroke, benefit usually outweighs risk
3. Contrast allergy history — premedication if previous mild reaction; avoid if severe prior anaphylaxis
4. Metformin — hold for 48 hours after contrast if eGFR < 60
5. Establish IV line patency — extravasation must be avoided (high injection rate)
6. Explain procedure — reassure regarding warmth/flushing sensation from contrast
7. Patient lies supine and still — head in headrest/immobiliser
8. No breath-hold required (unlike cardiac CT) — the brain does not move with breathing

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9. Scan Protocol — Step by Step

Step 1: Non-Contrast CT (NECT)

• Standard axial NECT of the brain first — mandatory
• Purpose: exclude haemorrhage before contrast, assess ASPECTS score, identify hyperdense vessel sign
• Parameters: 120 kVp, standard dose, 5mm slices, brain/bone window

Step 2: CT Angiography (CTA)

• Performed as part of multimodal CT in most acute stroke protocols
• Contrast bolus (60–80 mL at 4–5 mL/sec) with bolus tracking trigger on aortic arch
• Covers arch of aorta to vertex — assesses for large vessel occlusion (LVO)

Step 3: CT Perfusion Acquisition

Step 4: Image Acquisition Sequence

Inject contrast IV (4-6 mL/sec)
        ↓
Start CT acquisition at 4-6 sec delay
        ↓
Repeated CT passes over same brain levels
  → every 1-2 seconds for 40-60 seconds
        ↓
Generates ~40-60 time-points per level
        ↓
Time-density curve built for every pixel

Step 5: Post-Processing

1. Motion correction — automated realignment of all time-point images
2. Vessel identification — AIF selected (usually MCA or ACA), venous output function (VOF) selected from superior sagittal sinus
3. Deconvolution — mathematical calculation of CBF, CBV, MTT, Tmax for every pixel
4. Threshold application — software applies validated thresholds:
   • Core: CBF < 30% of contralateral (or CBF < 10 mL/100g/min)
   • Penumbra: Tmax > 6 seconds
5. Colour map generation — colour-coded parametric maps displayed
6. Mismatch calculation — penumbra volume ÷ core volume → mismatch ratio

Step 6: Reporting

• Core infarct volume (mL)
• Penumbra volume (mL)
• Mismatch ratio and mismatch volume
• Presence/absence of large vessel occlusion (from CTA)
• Any haemorrhagic transformation
• Collateral flow assessment

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10. Stroke Treatment Criteria Using CTP (DAWN / DEFUSE-3)

"CT perfusion within 6–16 hr: core infarct volume ≤70 mL, core-penumbra mismatch volume ≥15 mL, core-penumbra mismatch ratio ≥1.8" — Washington Manual of Medical Therapeutics

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11. Radiation Dose

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12. Limitations and Pitfalls

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13. Comparison of CT Perfusion vs. MR Perfusion

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14. Summary Table

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• Grainger & Allison's Diagnostic Radiology, Ch. 56 (Acute Stroke Imaging)
• Harrison's Principles of Internal Medicine, 22nd Ed., Ch. 434
• Schwartz's Principles of Surgery, 11th Ed., Ch. 42
• Washington Manual of Medical Therapeutics
• Rosen's Emergency Medicine, Ch. 14

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Myocardial Imaging in CT — In Detail

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1. Overview and Definition

1. Morphology and structure — wall thickness, mass, volume
2. Global and regional systolic function — ejection fraction, wall motion
3. Myocardial perfusion — stress CT perfusion (CTP), detection of ischaemia
4. Myocardial viability and tissue characterisation — infarct, fibrosis, oedema
5. Physiological significance of stenosis — CT-derived FFR (FFR-CT)

"Stress CT myocardial perfusion may be used for evaluating myocardial perfusion for the detection of functionally significant coronary artery disease." — Grainger & Allison's Diagnostic Radiology

"Evolving new applications for the adjudication of the hemodynamic significance of stenosis such as CT myocardial perfusion imaging and FFR-CT are now being used in clinical practice." — Fuster and Hurst's The Heart, 15th Ed.

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2. Why CT for Myocardial Imaging?

• Excellent spatial resolution (< 0.5 mm in-plane) — superior to nuclear or echo
• True 3D volumetric dataset — any plane can be reconstructed
• Fast acquisition — full heart in 5–15 seconds
• Ability to combine anatomy (CCTA) + perfusion (CTP) + function in a single session
• Limitation: ionising radiation, iodinated contrast, heart rate dependency

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3. Exam Diagram

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MODULE A: CT Assessment of Ventricular Morphology and Function

3A.1 Acquisition Requirements

• ECG-gated MDCT — mandatory to freeze cardiac motion
• Two acquisition modes:

• For LV function alone: low-dose cine CT throughout cardiac cycle suffices
• Iodinated contrast is required (except for CAC scoring)

3A.2 Standard Reconstructed Views

Short-axis views (basal, mid, apical):
Each ring divided into 6 (basal/mid) or 4 (apical) segments
+ Apex = segment 17

Coronary territory assignment:
  LAD  → anterior, anteroseptal, apical segments
  RCA  → inferior, inferoseptal segments
  LCx  → lateral, inferolateral segments

• Short-axis (SA) — most important; perpendicular to LV long axis
• Two-chamber (vertical long axis)
• Four-chamber (horizontal long axis)
• Three-chamber (LV outflow tract view)

3A.3 Ventricular Volume and Ejection Fraction

• End-diastolic volume (EDV) and end-systolic volume (ESV) are measured from contoured endocardial borders
• EF = (EDV − ESV) ÷ EDV × 100
• CT is highly accurate for volumetric analysis — more so than 2D echo
• LV mass = (epicardial volume − endocardial volume) × myocardial density (1.05 g/mL)

3A.4 Regional Wall Motion Assessment

• From functional cine reconstruction, systolic wall thickening and inward endocardial motion are assessed per segment
• Wall motion abnormalities (hypokinesis, akinesis, dyskinesis) in a coronary distribution = ischaemic aetiology
• Ventricular thinning + wall motion abnormality = scar / prior MI

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MODULE B: CT Myocardial Perfusion Imaging (CT-MPI / CTP)

3B.1 Principle

• A coronary stenosis may not limit blood flow at rest (autoregulation compensates)
• Pharmacological vasodilators (adenosine, dipyridamole, regadenoson) maximally dilate normal microvessels → increase flow in normal territories 3–5× above baseline
• Stenosed arteries cannot match this → relative hypoperfusion = perfusion defect appears on CT as hypoattenuation

3B.2 Two Types of CT Perfusion

3B.3 Protocol

Step 1: Rest CCTA (coronary CT angiography)
           ↓
        10–30 minute washout interval
        (allow contrast to clear + vasodilator to wear off)
           ↓
Step 2: Pharmacological stress
        → Adenosine 140 µg/kg/min IV × 3 min, OR
        → Regadenoson 0.4 mg IV bolus
           ↓
Step 3: IV contrast (40–60 mL at 5 mL/sec) injected
           ↓
Step 4: Stress CTP acquisition during peak myocardial enhancement
        → Timed bolus tracking or fixed delay (~30 sec post-injection)
           ↓
Step 5: Post-processing — short-axis colour maps generated
        → Perfusion defect = dark area (↓ HU) during stress
        → Normal myocardium = bright (↑ HU)

3B.4 Acquisition Parameters for Stress CTP

3B.5 Image Interpretation

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MODULE C: FFR-CT (CT-Derived Fractional Flow Reserve)

3C.1 Definition and Principle

• Pa = aortic (coronary inlet) pressure
• Pd = distal coronary pressure beyond the stenosis
• Normal FFR = 1.0; haemodynamically significant stenosis = FFR ≤ 0.80

3C.2 How FFR-CT Works

Standard resting CCTA
         ↓
Coronary artery segmentation (automated)
         ↓
3D mesh model of coronary tree built
         ↓
Computational fluid dynamics applied:
  → Patient-specific microvascular resistance assigned
  → Hyperaemic conditions simulated mathematically
         ↓
Pressure gradient calculated at every point
         ↓
FFR displayed as colour map along coronary tree
  Red/orange = FFR ≤ 0.80 (ischaemia-causing)
  Green = FFR > 0.80 (non-significant)

3C.3 Performance Data

• Nonobstructive CAD found in only 12% of FFR-CT guided patients who proceeded to ICA vs 73% in standard care
• 61% of CCTA/FFR-CT patients had ICA cancelled with no adverse events
• Significantly less expensive and better QoL improvement

3C.4 When to Use FFR-CT

• Intermediate coronary stenosis on CCTA (40–70% stenosis) where functional significance is unclear
• Avoids need for invasive angiography or stress testing
• Particularly useful for 50–69% stenoses where CTA alone performs poorly

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MODULE D: Myocardial Tissue Characterisation by CT

3D.1 CT Myocardial Infarction Detection

• Iodinated contrast behaves similarly to gadolinium in infarcted tissue
• In scarred/fibrotic myocardium, extracellular space is expanded → contrast pools → delayed hyperattenuation on CT at 5–15 minutes post-injection
• Subendocardial LIE in a coronary distribution = ischaemic scar (analogous to LGE-CMR)
• Transmural LIE = extensive infarction

3D.2 CT Appearance of Myocardial Conditions

3D.3 Myocardial Fat

• Fat appears as very low HU (−50 to −100 HU) within myocardium
• Seen in: lipomatous metaplasia of old infarct, arrhythmogenic right ventricular cardiomyopathy (ARVC), cardiac lipoma
• ARVC: fatty replacement of RV free wall — CT highly sensitive

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MODULE E: Cardiac CT for Structural Heart Disease

3E.1 Pre-TAVR CT Protocol

• Aortic annulus measurement — true short-axis plane, measured in systole
  • Annular area, perimeter, short and long diameters
  • Used for prosthetic valve sizing
• Aortic root anatomy — sinus of Valsalva dimensions, coronary ostia heights
• Left ventricular outflow tract (LVOT) assessment
• Iliofemoral access route assessment for catheter delivery
• Calcium scoring of aortic valve and LVOT

3E.2 Other Structural Uses

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MODULE F: Comparison of Cardiac CT with Other Modalities

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MODULE G: Protocols Summary — Complete Cardiac CT Study

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MODULE H: Radiation Dose Summary

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Limitations of CT Myocardial Imaging

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Summary Table

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• Grainger & Allison's Diagnostic Radiology, Ch. 12 & 15
• Fuster and Hurst's The Heart, 15th Ed., Ch. 3 & 6
• Harrison's Principles of Internal Medicine, 22nd Ed., Ch. 248
• Goldman-Cecil Medicine, Ch. 44