Liver-Spleen Imaging (Tc99m-sulfur colloid)

Objectives

Explain the rationale and clinical indications for liver-spleen scintigraphy:
- Evaluate the function of the reticuloendothelial system (RES).
- Assess Kupffer cell uptake in the liver and splenic macrophage function.
- Complement anatomical imaging techniques such as CT, MRI, and ultrasound.
- Provide functional information when structural imaging is equivocal.
- Recognize that it is less commonly used today but remains valuable for specific clinical indications.
Describe patient preparation and imaging protocol:
- Minimal preparation is required with no strict fasting requirement (subject to facility protocols).
- Inform patients about procedure duration, absence of pain, and the need to remain still.
- Screen for pregnancy in women of childbearing age.
- Remove metal artifacts from the imaging field.
Identify normal uptake patterns and anatomic variants:
- Normal biodistribution should show the liver with the highest uptake (80-90%), followed by the spleen (5-10%).
- Recognize benign normal variants like Riedel's lobe that show normal uptake patterns.
Recognize abnormal findings:
- Identify focal defects, diffuse liver disease, and colloid shift.
- Understand the implications of these findings on the diagnosis.
Discuss artifacts and diagnostic limitations:
- Understand factors that can impact image quality and interpretation such as motion artifacts, attenuation, and previous scans.

Rationale and Role in Clinical Practice

The imaging evaluates the function of the reticuloendothelial system (RES), which plays a crucial role in the body's immune response and filter processes.
It assesses Kupffer cell uptake in the liver, which is vital for understanding liver function, alongside analyzing splenic macrophage activity.
It provides functional information that can complement anatomical imaging techniques (CT, MRI, Ultrasound) when those studies lack clarity.
Although its use has diminished, scintigraphy retains value for specific clinical conditions.

Cell Types Involved in Liver-Spleen Imaging

Hepatocytes: Primary functional cells in the liver.
Reticuloendothelial cells: Survey the body for pathogens and are prominently featured in the liver and spleen.
- Kupffer cells: Specialized macrophages located in the liver.
Reticular cells: Primary cells in the spleen and bone marrow involved in immune response and filtration.

Clinical Indications for Liver-Spleen Imaging

Evaluation of abnormal liver function tests.
Differentiation between focal (localized) versus diffuse (widespread) liver disease.
Assessment of hepatosplenomegaly (enlarged liver and spleen).
Evaluation of portal hypertension and cirrhosis.
Detection of functional asplenia (absence of spleen function).
Characterization of space-occupying lesions in the liver or spleen.

Contraindications and Limitations

Few absolute contraindications exist, but pregnancy requires thorough justification before proceeding with imaging.
The modality has limited spatial resolution compared to CT or MRI, which affects its diagnostic accuracy.
Small lesions (less than 2 cm) may not be detected effectively, limiting its use for certain cases.
It cannot definitively characterize lesions, necessitating complementary imaging for detailed assessments.

Patient Preparation

Minimal preparation is required for the patient before imaging.
There is no strict fasting requirement, though checking with facility protocol is advised.
Patient education is crucial regarding the duration of the procedure, reassurance of no pain experienced during the scan, and the need for them to remain still.
Pregnancy screening is essential for women of childbearing age to prevent risks.
Removal of metal artifacts from the imaging field ensures clearer images.

Radiopharmaceutical: Tc-99m Sulfur Colloid

The primary radiopharmaceutical used is 99mTc-sulfur colloid (Tc-99m SC), which is characterized by:
- Particle size: Ranges from 0.1 to 1.0 microns.
- Phagocytized by: Kupffer cells in the liver, reticular cells in spleen, and bone marrow.
- Typical adult dose: Administered dose is 185–370 MBq (5–10 mCi).
- Rapid blood clearance: Imaging starts 10–15 minutes following injection.

Normal Biodistribution

Liver: Receives 80–90% of the activity, predominantly in the right lobe.
Spleen: Receives 5–10% of the activity.
Bone marrow: Accounts for approximately 5% of activity (usually not visualized clearly in planar imaging).
Kidneys, bowel, or bladder: Exhibit no significant activity.
There should be uniform uptake throughout hepatic parenchyma without focal defects.

Imaging Procedure

Patient positioning: Supine on the imaging table.
Acquisition of multiple planar views:
- Anterior, posterior, right lateral, and left lateral views are standard.
- Optional oblique views provide better localization of lesions.
- Use external markers to delineate liver borders accurately.
SPECT or SPECT/CT can be added for complex cases to enhance imaging detail.

Normal Liver-Spleen Scan

Homogeneous liver uptake with smooth contours.
The size relationship should display that the liver is larger than the spleen.
The spleen is visible in the left upper quadrant.
There should be no focal defects or areas of increased or decreased uptake.
Bone marrow uptake should be minimal or not visualized at all.

Normal Variant: Riedel's Lobe

Definition: An elongated, tongue-like extension of the right hepatic lobe that projects inferiorly.
May reach the iliac crest, characteristic of a benign anatomical variant that is more common in women.
Can mimic hepatomegaly or a right-sided mass upon palpation.
Should demonstrate normal uptake and smooth contour upon imaging.

Abnormal Patterns Overview

Single focal defect (cold spot): Represents an area of absent or decreased RES function.
Multiple focal defects: May indicate multiple concurrent diseases.
Diffuse hepatic disease: Characterized by heterogeneous uptake.
Colloid shift: Reflects decreased liver uptake with increased splenic or bone marrow uptake.
Spleen abnormalities: May include enlargement, nonvisualization, and focal defects.

Single Focal Defect

Represents an area of absent or decreased RES (reticuloendothelial system) function.
Differential diagnosis should include:
- Metastasis
- Primary tumor
- Cyst
- Abscess
- Hematoma
Lesions that are greater than 2 cm are more reliably detected.
Scans cannot distinguish between benign and malignant causes, hence requiring correlation with CT/MRI and clinical history for a definitive diagnosis.

Multiple Focal Defects

Suggests a multifocal disease process.
Common causes include:
- Metastatic disease
- Multifocal hepatocellular carcinoma
- Multiple abscesses, cysts
- Nodular regenerative hyperplasia
Notably, while the pattern indicates a disease process, it cannot determine etiology.
Biopsy and further imaging are often necessary for accurate diagnosis.
Example of patient: those with colon metastases may show this pattern.

Diffuse Hepatic Disease

Characterized by heterogeneous or decreased uptake throughout the liver.
The liver may appear enlarged or potentially normal in size.
Possible causes include:
- Cirrhosis
- Hepatitis
- Fatty liver disease
- Infiltrative diseases
Often associated with irregular liver contours.
Possible colloid shift may also occur, indicating severe liver dysfunction.

Colloid Shift (Critical Concept)

Defined as a state of decreased liver uptake coinciding with increased spleen and bone marrow uptake.
It indicates a redistribution of RES function away from the diseased liver toward other sites.
Recognized as a sign of severe hepatocellular dysfunction.
This may occur in conditions such as:
- Cirrhosis
- Severe hepatitis
- Effects related to chemotherapy
Colloid shift may be reversible if the underlying condition improves.

Hepatosplenomegaly

Characterized by an enlarged liver and spleen with increased splenic uptake.
The liver-to-spleen uptake ratio may be reversed or equalized.
Causes may include:
- Portal hypertension
- Storage diseases
- Hematologic disorders
It may be associated with a colloid shift, which necessitates careful clinical correlation for precise diagnosis.

Spleen Abnormalities

Nonvisualization can indicate conditions such as:
- Functional asplenia
- Splenectomy
- Splenic infarction
An enlarged spleen may suggest issues concerning:
- Portal hypertension
- Storage diseases
- Lymphoma
Focal splenic defects can result from:
- Infarcts, abscesses, or metastases (though rare).
Accessory spleen: Recognized as a normal variant indicating a small focus of functional splenic tissue.
It is imperative to evaluate the spleen on every liver-spleen scan to identify these abnormalities effectively.

Sensitivity, Specificity, and Limitations

The imaging modality possesses high sensitivity for identifying diffuse liver diseases and colloid shift.
There is lower sensitivity for detecting small focal lesions (less than 2 cm).
It is not effective for lesion characterization, differentiating benign from malignant.
CT and MRI are superior options for anatomical detail and characterization of small lesions.
The liver-spleen scan is best utilized alongside other imaging modalities to enhance diagnostic accuracy.

Artifacts and Pitfalls

Bowel activity from recent meals can mimic hepatic uptake, leading to false interpretations.
Motion artifacts may blur images and create apparent defects on the images.
Attenuation from overlying structures (e.g., breast tissue, ribs) can obscure the liver and spleen details.
A poor bolus/infiltrated injection can result in low-quality images.
Previous nuclear medicine scans may interfere with current imaging results.
External contamination can lead to false hot spots on images.

Summary: Key Takeaways

Liver-spleen imaging evaluates the RES function using 99mTc-sulfur colloid.
A normal scan shows homogeneous liver uptake, where the liver is larger than the spleen, with minimal marrow uptake.
Recognize Riedel's lobe as a benign normal variant in imaging.
Key abnormal patterns to identify include: focal defects, diffuse disease, and colloid shift.
Colloid shift is a hallmark of severe liver dysfunction and is an especially crucial concept in the evaluation of liver-spleen imaging.