Cardiac Innervation Imaging with mIBG

Cardiac Innervation Imaging with mIBG

Objectives

• Explain: mIBG mechanism and sympathetic nerve uptake.

• Identify: clinical indications for cardiac mIBG.

• Describe: the imaging protocol and patient preparation for HMR and washout rate.

• Calculate/interpret: relevant metrics related to mIBG imaging.

Anatomy - The Autonomic Nervous System

• Autonomic nervous system: Consists of:

  • Sympathetic: Responsible for the "fight or flight" response.

  • Parasympathetic: Responsible for the "rest and digest" functions.

• Pathway:

  • Brain → spinal cord (T1-T4) → postganglionic fibers → heart ventricles.

• Neurotransmitter Used: Norepinephrine.

• Mechanism of Norepinephrine Use:

  • The Uptake-1 pathway recycles norepinephrine back into nerve terminals.

• KEY POINT: mIBG utilizes the same uptake-1 pathway for its action as norepinephrine does.

What is Cardiac Autonomic Dysfunction?

• Definition: The disruption of the normal balance between sympathetic and parasympathetic control of the heart.

• Causes of Cardiac Autonomic Dysfunction:

  • Intrinsic Problems: Nerve abnormalities within the autonomic system itself.

  • Extrinsic Problems: Diseases or conditions that affect nerves externally.

• Consequences of Dysfunction:

  • Irregular heart rate

  • Blood pressure instability

  • Arrhythmias

  • Impaired relaxation of the heart.

Introduction to mIBG

• Full name: Metaiodobenzylguanidine, labeled with I-123.

• Key Characteristics:

  • It is an analog of norepinephrine, meaning it has a similar structural formula.

  • Labeled with I-123, a gamma emitter used in imaging.

  • It uses the same uptake-1 pathway as norepinephrine, allowing for effective heart imaging.

  • Stored in nerve vesicles, but it does NOT have any physiologic effect on the heart.

• Initial Development Purpose: Originally developed for imaging of the adrenal gland and neuroendocrine tumors.

How mIBG Imaging Works

• Normal Uptake: Indicates high uptake in regions with intact sympathetic innervation, significantly higher in the heart than in surrounding structures (like the mediastinum).

• Abnormal Patterns Indicating Dysfunction:

  • Reduced Uptake: May suggest denervation or dysfunction in the region being examined.

  • Increased Washout: Indicates impaired storage or function of the norepinephrine pathway.

• mIBG Distribution in the Body:

  • Salivary glands, thyroid, heart, liver, spleen, adrenal glands, kidneys, bladder.

Clinical Indications for Cardiac mIBG Imaging

• Heart Failure: Important for assessment and prognosis of the condition.

• Suspected Cardiac Dysautonomia: To evaluate autonomic function.

• Parkinson's Disease: Useful for differentiation from other similar syndromes.

• Dementia with Lewy Bodies: mIBG can help in diagnosis and management.

• Diabetic Cardiac Neuropathy: Important in assessing nerve damage due to diabetes.

• Post-Heart Transplant Monitoring: For evaluating cardiac function post-surgery.

• Ischemic Heart Disease: Helpful in identifying autonomic dysfunctions associated with ischemia.

Patient Preparation - Medications

• Medications to Withhold:

  • Tricyclic Antidepressants: 24-48 hours before imaging.

  • Antipsychotics: 24 hours to 1 month.

  • Calcium Channel Blockers: 24-48 hours before imaging.

  • Decongestants/Sympathomimetics: 24-48 hours prior.

  • Beta-agonists: 24 hours before.

• Thyroid Blocking (MANDATORY):

  • SSKI or Potassium Perchlorate: To be withheld 1 hour prior to injection to protect the thyroid from free I-123, with the exception of post-thyroidectomy patients.

  • Dietary Restrictions: Avoid chocolate and blue cheese for approximately one week prior to the exam.

  • Hydration and Voiding: Important for patient management prior to imaging.

  • Pregnancy/Lactation Assessment: Recommend discontinuing breastfeeding for 48 hours to 6 days before the procedure.

Radiopharmaceutical Dosing

• Standard Adult Dose:

  • 10 mCi (370 MBq) of I-123 mIBG for patients aged 16 years or older, or those weighing 70 kg or more.

• Pediatric Dosing:

  • 0.14 mCi/kg (5.2 MBq/kg) with a minimum dosage of 1 mCi.

• Administration Technique:

  • Administered via slow intravenous injection over 1-2 minutes, followed by a saline flush to ensure complete delivery.

Imaging Protocol - Timing

• Option 1: Early + Delayed Imaging

  • Early Imaging: 15 minutes after injection.

  • Delayed Imaging: 3 hours 50 minutes to 4 hours after injection.

• Option 2: Delayed Imaging Only

  • Delayed Imaging: 3 hours 50 minutes to 4 hours post-injection.

  • Advantages: More consistent outcomes which give a full picture of neuronal function.

Imaging Protocol - Equipment Setup

• Camera Requirements: Large field-of-view gamma camera.

• Collimator: Low-energy, high-resolution parallel hole.

• Energy Window: 20% window centered at 159 keV.

• Matrix: 256 × 256 (preferred) or 128 × 128 (acceptable).

• Patient Positioning: Supine (women should remove bras).

• View Configuration: Anterior chest view for 10 minutes, ensuring the heart and mediastinum are included.

• Positioning Details:

  • Patient arms should be positioned upwards to allow a 360° orbit and perform 120 projections (60 per head if dual).

  • Each step should last 30 seconds, and 128 × 128 matrix is required.

• Gating Requirements: No gating required for planar SPECT scanning.

Image Processing and Quantification - Overview

• Two Main Quantitative Measurements:

  1. Heart-to-Mediastinum Ratio (HMR):

  • Compares heart uptake to the background exposure, and is the most critical measurement.

  1. Washout Rate (WR):

  • Measures the rate at which the tracer leaves the heart, requiring both early and delayed images for calculation.

Heart-to-Mediastinum Ratio (HMR)

• ROI (Region of Interest) Calculations:

  • Cardiac ROI: Can be circular or freehand, must encompass the entire heart, and calculate mean counts per pixel.

  • Mediastinal ROI: A rectangular shape in the upper mediastinum, should be ≥4 pixels below the clavicular heads while avoiding thyroids, muscles, and lungs; calculate mean counts per pixel.

• Formula for HMR Calculation:
  $HMR = \frac{Mean \, Heart \, Counts/Pixel}{Mean \, Mediastinal \, Counts/Pixel}$

• Example Calculation:

  • If mean heart counts = 45 and mean mediastinal counts = 20, then:

  • $HMR = \frac{45}{20} = 2.25$

• Normal Delayed HMR Range: 1.9-3.0.

Interpreting HMR Values

• Normal Values (1.9-3.0): Indicates intact postganglionic sympathetic innervation and good uptake/retention in nerve terminals.

• Low Values (<1.9):

  • Suggests reduced sympathetic innervation, indicating possible denervation or dysfunction which correlates with poorer prognosis in heart failure.

• Factors Affecting HMR:

  • Patient’s age, gender, cardiac function, activity level, ROI placement, and camera settings can all influence HMR results.

Washout Rate (WR) Calculation

• Requirements for Calculation: Must include both early and delayed images.

• Formula for WR Calculation:
  $WR = \frac{(Early \, Heart - Early \, Mediastinum) - (Delayed \, Heart - Delayed \, Mediastinum)}{(Early \, Heart - Early \, Mediastinum)} \times 100$

• Output: Expressed as a percentage.

• Normal WR Range: 21% to 37%.

• Elevated WR Indications: Suggests impaired storage and increased sympathetic activity.

SPECT Reconstruction and Analysis

• Advantages of SPECT:

  • Provides better contrast resolution and facilitates 3D visualization of tracer uptake.

  • Allows for segmental analysis capacity by dividing the left ventricle into segments (typically 17 segments).

• Segmental Scoring System:

  • Score each segment from 0 (normal uptake) to 3-4 (absent uptake).

  • Total scores provide assessment of defects, useful for localizing regional dysfunction (e.g., post-infarct).

Visual Interpretation: Normal Cardiac mIBG

• Expected Normal Findings:

  • Strong, uniform tracer uptake throughout the left ventricle.

  • Heart uptake is visibly greater than the mediastinum.

  • Symmetrical appearance with clear cardiac borders.

• Normal Physiologic Uptake Also Seen In: Salivary glands, thyroid, lungs, liver, spleen, kidneys, and bladder.

Abnormal Patterns on Cardiac mIBG

• Globally Reduced Uptake:

  • Heart appears faint, similar to a mediastinal background.

  • Examples include conditions like Parkinson's disease, heart failure, and generalized dysautonomia.

• Focally Reduced or Absent Uptake:

  • Indicates a regional defect in uptake.

  • Examples include post-myocardial infarction or ischemic heart disease.

• Increased Washout on Delayed Images:

  • Heart appears significantly fainter post-delay compared to early imaging, suggesting storage dysfunction.

Clinical Application: Heart Failure

• mIBG Findings in Heart Failure:

  • Reduced HMR, often less than 1.6.

  • Increased washout rate, commonly greater than 40%.

• Clinical Correlation:

  • Lower HMR values correlate with disease severity and prognosis.

  • Lower HMR indicates higher mortality risks and higher likelihood of cardiac events.

  • Helps to guide treatment decisions and risk stratification for patients.

Ensuring High-Quality mIBG Scans

• Patient-Related Factors:

  • Verify that thyroid blocking was adequately given.

  • Confirm patient medication status is appropriate.

  • Document any relevant medical history that may affect results.

• Technical Factors:

  • Ensure proper energy window (159 keV ± 20%).

  • Use correct collimators (Low-Energy High-Resolution, LEHR).

  • Achieve adequate count statistics for robust imaging.

  • Ensure precise timing relative to injection for consistent imaging.

Common Pitfalls and How to Avoid Them

• Medication Interference:

  • Ensure patients stop interfering medications to avoid falsely low uptake results.

• Thyroid Uptake Issues:

  • Confirm that adequate thyroid blocking was provided; intense thyroid activity can complicate interpretation.

• ROI Placement Errors:

  • Proper positioning of mediastinal ROI to prevent placement too close to heart or thyroid; ensure cardiac ROI encompasses the entire heart adequately.

• Patient Motion Concerns:

  • Monitor patient motion, particularly during SPECT imaging, to avoid blurring and artifacts affecting quality.

Key Takeaways

• Overall Purpose of mIBG: mIBG is a norepinephrine analog used to visualize cardiac sympathetic innervation effectively.

• Understanding HMR Values: Normal hearts show high uptake with HMR values ranging from 1.9 to 3.0; dysfunction indicates low uptake.

• Prognostic Value: Critical in assessing prognosis in heart failure and diagnosing neurological conditions.

• Significance of Patient Preparation: Crucial to perform effective thyroid blocking, rationalize medication cessation, and ensure hydration.

• Standard Dosage and Imaging Timing: Standard dose is 10 mCi for adults, with imaging ideally performed at 4 hours post-injection.

• Quantitative Consideration: HMR measurement is the most important quantitative assessment to make.

• Reproducibility in ROI Placement: Use a systematic approach to ensure consistent ROI placement for valid and reproducible results.