Advanced Imaging Techniques for Pulmonary Disease: Types and Uses

Introduction

Identification of the underlying pathology is the basic stage of the treatment procedure. But, due to the complex nature of the diseases and anatomical variations, it is often difficult for researchers to pinpoint the diagnosis. Modern diagnostic tests play a vital role in diagnosing disease conditions. Imaging procedures are really helpful as these procedures help in the visualization of the underlying structures. Advancements in imaging technique is a giant step toward the diagnosis of lung diseases.

What Are the Traditional Imaging Techniques?

The traditional imaging techniques used in the detection of lung diseases are:

1. Radiography: Radiography is the most commonly used diagnostic procedure. It is also one of the most economical techniques.

2. Bronchoscopy: In this procedure, visualization of the internal structure of the lungs can be done. This technique was developed by S. Ikeda in the 1960s. A flexible fiberoptic scope is inserted into the windpipe and in the lung to observe the lung structures.

3. Computerized Tomography (CT): In this procedure also, radiographic images of the lung are taken. Unlike the traditional X-ray image, the source moves around the body in a circular or spiral motion to take images.

4. Magnetic Resonance Imaging (MRI): In this technique, strong magnetic fields and radio waves are used to obtain lung images.

Modern imaging techniques are based on these traditional techniques. These are:

1. Positron emission tomography (PET).

2. Dual-energy CT.

3. High-resolution computed tomography (HRCT).

4. Ventilation CT.

5. MRI using noble gases.

6. Endosonography.

What Is Positron Emission Tomography (PET)?

In this technique, cellular functioning, tissue morphology, cellular metabolism, and blood floor can be measured by the use of radiolabeled drugs. F-18 fluorodeoxyglucose (FDG) is a radiolabeled drug used in this procedure. This radiotracer is injected in the vein around one hour before the procedure. FDG emits positrons. These positrons combine with electrons and, through the annihilation process, produce photons. These emitted photons travel exactly in opposite directions. The PET camera can detect these emissions, which can be used to produce images.

The malignant cells have high metabolic activity. As a result, FDG uptake by the malignant cells by glucose transporter molecules is very high. PET-CT scans can be used in various cases of lung cancers:

1. Identification and characterization of solitary lung nodules can be done.

2. Accurate staging of non-small cell lung cancer can be done.

3. It can be used to detect small-cell lung cancers.

4. Differentiation between benign and malignant lung cancer can be done.

5. Evaluation of the treatment response can be done.

What Is Dual-Energy CT?

In Conventional computerized tomography (CT), single X-ray spectra are used to create images. In dual-energy CT ((DECT), two different spectra of the X-rays are used. Changes in the tissue response in the high-energy and low-energy spectra are studied to evaluate tissue characterization. Based on techniques, dual-energy CT can be of two types:

1. Dual source DECT.

2. Single source DECT.

This dual-energy computerized tomography can be used to quantify various substances in the tissue to evaluate functioning. For these purposes, contrast mediums are used that are injected prior to the procedure via catheter through the intravenous route. Iodine contrast medium is used for lung lesions. The quantification of iodine in lung tissue helps in the following ways:

1. Perfusion of the lung blood vessels can be evaluated.

2. The presence of a thrombus in the blood vessels can be identified.

3. This can be used to detect tissue characteristics in lung emphysema cases.

What Is High-Resolution Computed Tomography (HRCT)?

In this procedure, thin slices of the tissue images are obtained with the help of a high-energy spectrum. Increased speed of the tube rotation, increased detector sensitivity, and more

rows of detectors are responsible for better image quality and thinner slices. These improvements also help to reduce breathing artifacts during the imaging procedure. The average slice thickness in this procedure is between around 0.625 to 1.25 millimeters. This helps in the three-dimensional reconstruction of lung structures with less amount of radiation exposure. The utility of such procedures are:

1. It helps in the detection and characterization of emphysema.

2. Assessment of airways can be done through HRCT.

3. Fibrosis of the lung can be evaluated.

4. Conditions like sarcoidosis and chronic pulmonary vascular disease can also be identified.

What Is Ventilation CT?

This procedure is also known as 4D CT or perfusion scan. In this technique, functional lung mapping is done with the help of radioactive substances. For ventilation scanning, 99mTc-diethylenetriaminepentaacetic acid (DTPA) is used in the form of an aerosol. The diameter of the aerosol is 4.5 micrometers with a half-life of 6 hours. A maximum dose of 25 to 35 millicuries of this radiopharmaceutical is administered via nebulizer. Inert gasses used for ventilation scanning are 133Xe and 81mKr. The maximum dose of such gasses is 5- 20 millicuries. In perfusion scintigraphy, 9mTc-macro aggregated albumin (99mTc-MMA) with a particle size of 10 to 100 micrometers is injected intravenously with a dose of 40 to 150 mega-becquerel.

The use of these procedures are:

1. Identification of arteriovenous malformations can be done.

2. Pulmonary embolisms can be identified.

3. Incidences of cardiovascular shock, alveolar collapse, and pulmonary infections such as pneumonia can be diagnosed.

What Is MRI Using Noble Gases?

The lung structure is filled with gaseous substances, and the concentration of hydrogen ions is less. Because of these complexities, the reconstruction of lung images through magnetic resonance imaging is difficult. Inert noble gasses are used to overcome these limitations. Hyperpolarized noble gasses such as Xenon are used in such cases. These gasses help assess lung ventilation and functional efficiency.

However, availability and cost factors are the main drawbacks of this technique. Oxygen-enhanced MRI (OE-MRI) is now being used to overcome these drawbacks. In this procedure, 100 percent oxygen concentration is used for the determination of lung function.

What Is Endosonography?

This is an ultrasonography technique used in lung cancers. Unlike traditional bronchoscopy, this technique utilizes ultrasonography via two different approaches. When ultrasonography of the lung structures is done through the esophagus, it is known as endoscopic ultrasound. When the ultrasonography is done through the respiratory tract, it is called endo-bronchial ultrasound. The uses of this procedure are:

1. Localization of the lung lesions and evaluation of the lung lesions in the real-time view.

2. Identification of the lung and respiratory lesions and assessment of the extension.

3. Tissue sampling for the biopsy procedure is possible.

4. Accurate staging of lung cancer is possible,

5. Assessment of metastasis is possible.

Conclusion:

Diagnosis is the most important step of the treatment procedure. Modern diagnostic procedures are helpful in determining lung cancer at an early stage. Also, the staging of cancer and its progression also can be evaluated. The extension of lung damage and its functional efficiency can also be diagnosed using these procedures.