Imaging Modalities in Chest Tumors: Role and Techniques

Devasenathipathy  Kandasamy; Gh Mohammad Wani

Section 1

1Imaging Modalities

Imaging Modalities in Chest Tumors: Role and Techniques
Role of PET-CT in Chest Tumors
Artificial Intelligence and Radiomics: Basics and Applications2

Imaging Modalities in Chest Tumors: Role and TechniquesCHAPTER 1

Devasenathipathy Kandasamy,

Gh Mohammad Wani

INTRODUCTION

There are battery of investigations available to the radiologists to evaluate the chest diseases. Most commonly used modalities are plain radiography, ultrasonography (USG), computed tomography (CT), and magnetic resonance imaging (MRI). In each of the modalities there are different techniques which can be used to get the maximum information in the given clinical context. Hence, the complete understanding of the strengths and weaknesses of the modalities and various techniques is vital to maximize their utility. In the context of chest tumors, the primary role of imaging is detection, characterization, staging, and follow-up after treatment. Each modality has its own advantages and limitations which will be discussed below.

PLAIN RADIOGRAPHY (FIGS. 1 TO 4)

Role

Used as initial screening modality for respiratory complaints and guides further imaging
Not good for staging of tumors as it cannot assess lymph node, chest wall, or mediastinal invasion
Used in respiratory emergencies in intensive care unit (ICU) and bedside
Screening for metastases and complications related to disease and treatment
Follow-up of lung nodule to assess the doubling time.4

Techniques

Table 1 Radiographic techniques.

Technique	Comments
Chest radiograph: Posteroanterior (PA) view	Standard technique Most commonly performed Most of the measurements and signs described on radiographs apply to this view
Chest radiograph: Anteroposterior view	Performed when patient cannot be positioned for PA view such as in ICU, bedside and children
Chest radiograph: Penetrated view	Usually performed to better visualize mediastinal structures, calcification in the mass lesion and to look for rib erosion
Chest radiograph: Lateral view	Rarely performed-not a standard part of chest imaging except in few centers around the world CT has almost replaced this view
Lordotic view (Apicogram—if only the apices are included)	Specialized view to demonstrate the apices of lungs which are usually hidden behind the clavicles on routine views CT has almost replaced this view
Oblique view of chest	Rarely performed Useful in showing chest wall or pleural masses CT has almost replaced this view
Dual energy subtraction radiography	Two energy (high and low) rays are used to create three types of images (standard image, soft tissue selective image and bone selective image). By removing overlapping soft tissue or bony structure depending on the image type it is superior in demonstrating tiny areas of calcification, pleural lesions, soft tissue lesions and hilar structures

ULTRASONOGRAPHY

Role

There are evolving roles with newer indications appearing regularly, particularly important in children
Used to assess mediastinal lymph nodes and to guide sampling in children
Normal thymus/hyperplasia vs mediastinal mass in children (Figs. 1A and B)
Cystic versus solid differentiation (lung cysts, hydatid, etc.) (Figs. 2A to C)
May be the only modality required in several benign chest wall masses.

Techniques

Table 2 USG techniques.

Indication	Probe	Approach	Any special technique
Chest wall masses	Linear, high frequency 9–15 MHz	Direct	Standoff pad/gel
Mediastinal masses	Convex sector or endocavitary probe	Subxiphoid, suprasternal or parasternal	Insonate through the intercostal spaces in the parasternal region5
Pleural pathology (effusion, pneumothorax, mass)	Curvilinear or sector probe	Direct	Look at posterior costophrenic angle in erect position for minimal free effusion Look at nondependent regions for pneumothorax Look for pleural sliding to evaluate tumor infiltration of chest wall

Figs. 1A and B: Chest radiograph frontal view (A) of a child with asthma showed a mass like opacity in the anterior mediastinum. USG using a high resolution linear probe (B) was performed (arrow) to rule out a mass lesion which showed a soft tissue in the prevascular region with a typical starry sky appearance suggestive of thymus.

Figs. 2A to C: Chest radiograph of a patient (A) showing opaque hemithorax with ipsilateral shift of mediastinum. USG of the right lung (B) showing an area of dense calcification (thin arrow). These findings were also confirmed on CECT scan (C). The patient underwent right pneumonectomy and the lesion was diagnosed as carcinoid on histopathology.

Standard linear and convex probes are the most commonly used
Small footprint probes should be used if the acoustic window is small
Conventional gray scale USG with color Doppler wherever vascularity assessment is necessary
Elastography of pleura has been found to improve characterization of lesions
Contrast-enhanced ultrasound (CEUS) can add on to gray scale and color Doppler in the evaluation of masses.

MULTIDETECTOR COMPUTED TOMOGRAPHY

Role

It is the workhorse of thoracic imaging especially in the characterization and staging of tumors (Figs. 2 and 3).

Figs. 3A to C: Chest radiograph (A) of a patient showing a large mass lesion silhouetting the left cardiac and diaphragmatic border; (B and C) CECT scan of the patient (arrow) showing a large soft tissue mass arising from the anterior chest wall with a large extra- and intrathoracic component.

Figs. 4A to C: Chest radiograph (A) of a patient with left upper chest pain showing a thick-walled cavity (arrow) seen in the left upper lobe; (B and C) CECT scan confirmed the presence of cavitatory mass lesion with air fluid level. Sampling of the mass revealed squamous cell carcinoma of lung.

Ability to show the body structures in cross section, faster acquisition, and thin slice acquisition has revolutionized chest imaging (Figs. 4A to C).
Although many new techniques are currently available to reduce radiation dose, it is still a major concern.7

Techniques and their Indications

Table 3 CT techniques and indications.

CT scan technique	Indication
Noncontrast CT (NCCT)
NCCT (standard dose 120 kVp, 150 mAs)	Lung parenchymal evaluation Screening for metastasis Postchemotherapy and radiotherapy changes In patients with renal impairment
LDCT: Low-dose CT (90–120 kVp, 30–50 mAs)	Lung cancer screening Follow-up of lung parenchymal abnormalities
ULDCT: Ultra low-dose CT (80–100 kVp, ~20 mAs)	Follow-up of solid pulmonary nodule >2 mm
Contrast-enhanced CT (CECT)
CECT (standard dose 120 kVp, ~150 mAs)	Workhorse of chest tumor evaluation for characterization and staging (T, N and M) Can also evaluate lung parenchyma Multiplanar reformat and 3D volume rendering
Perfusion CT (Figs. 5 and 6)	Evaluation of first pass contrast kinetics Assessment of neoangiogenesis, response to therapy, etc. Patlak plot or deconvolutional models are commonly used mathematical models
Dual energy CT	Provides morphological and functional information of tumors Potential to show iodine-based perfusion maps
Cardiac/respiratory-gated CT	Useful to assess chest wall, diaphragmatic, and mediastinal invasion by demonstrating sliding of structures

Figs. 5A to C: Perfusion CT (A) of a patient with left upper lobe lung carcinoma showing an enhancing mass lesion (outlined arrow) with a central area of necrosis (thin arrow). The parametric maps showing blood flow (B) and blood volume (C), both of which are showing drastic reduction in the central necrotic area.

Figs. 6A to C: Perfusion CT (A to C) of a patient with right lung mass showing the role of blood flow and blood volume maps (B and C) in differentiating mass lesion from atelectatic lung which is otherwise challenging just based on gray scale image (A). The thin arrow shows the mass lesion whereas the outlined arrow shows the atelectatic lung.

Standard single phase CT protocols for chest tumors

Table 4 Standard single phase CT protocols for chest tumors.

Tube voltage	100 (<~80 kg) 120 (~80–113 kg) 140 (>113 kg)
mAs	130–200
Collimation of detector	Lowest possible on the scanner (e.g. 0.6)
Beam pitch	1.2
Gantry rotation time	0.5 seconds
Reconstruction slice thickness	1.2–1.5 mm
Image matrix	512 × 512
Radiation dose	3–8 mSv
Contrast	300 mg/mL iodine Nonionic contrast
Timing of scan	70 seconds after injection
Reconstructions	Lung window (soft tissue and high-resolution kernel) Mediastinal window Bone window

MAGNETIC RESONANCE IMAGING

Role

In patients with compromised renal function—noncontrast MRI is superior to noncontrast CT (NCCT).
Cardiac MRI is the modality of choice for suspected myocardial involvement by tumors (mediastinal masses).
Differentiation of thymic hyperplasia from thymoma using chemical shift imaging.
Equivocal chest wall, diaphragmatic or mediastinal invasion can be evaluated on cine MRI.
Chest wall masses without bony involvement on clinical/chest X-ray (CXR)/USG assessment—for their characterization and extent.9
Posterior mediastinal masses especially to evaluate spinal canal invasion.
Lung nodules—follow-up/characterization of likely benign nodules in children and young patients.
Brain metastases in patients with carcinoma lung are best evaluated with MRI.

Sequences and their Indications

A building-blocks approach is used when planning thoracic MRI.

Table 5 MRI sequences and indications.

Indications	Sequences
Chest wall mass	Fast spin-echo (FSE) T2 fat-saturated or short tau inversion recovery (STIR) T1 fat-saturated postcontrast
Mediastinal mass	FSE T2 fat-saturated or STIR T1 fat-saturated postcontrast
Lung nodules	Gradient recall echo sequences—2D or 3D, short- or ultra-short Spoiled gradient echo sequences Postcontrast: 3D gradient recalled T1 fat-saturated

Coils:
- For large intrathoracic masses—body coil (phased array)/cardiac coil
- Chest wall masses—surface coils.
Breath hold sequences whenever feasible, as respiratory and cardiac gating substantially increases examination times. However, respiratory gating is required for all nonbreath hold sequences. Cardiac gating is required depending on the location of the mass.
Methods of respiratory gating: External devices, e.g. pneumobelts, navigator sequences.
Some useful free-breathing sequences for patients unable to hold breath including young children: Radial k space-based sequences such as T2 PROPELLER/BLADE.
Contrast: Macrocyclic agents, e.g. gadobutrol (Gadovist), gadoterate (Dotarem) are preferred, especially in children.

MRI Techniques and Indications

Table 6 MRI techniques and indications.

MRI sequences	Indications
Conventional MRI	Basic characterization and local staging of masses especially posterior mediastinal lesions
Diffusion-weighted MRI (Figs. 7A to C)	To characterize cellular tumors Pleural deposits and lymph nodal involvement10
Perfusion MRI—time resolved/first pass magnetic resonance angiography (MRA) (Figs. 8A to C)	To evaluate pulmonary vasculature, e.g. pulmonary thromboembolism To generate perfusion parameters such as pulmonary blood flow (PBF) and blood volume (PBV)
Cine MRI	Useful to evaluate chest wall, diaphragmatic or mediastinal invasion
Cardiac MRI (double inversion recovery sequences, cine MRI)	For cardiac invasion

Figs. 7A to C: MRI with DWI (A to C) showing a large neurofibroma in the mediastinum (thin arrow) along with multiple tiny neurofibromas throughout the chest in a patient with neurofibromatosis.

With the advances in the imaging technology the morphologic imaging has been complimented by functional information which can be obtained from newer techniques. These functional information can be used to characterize the lesion, even predict the histopathology, prognosticate or to assess response to treatment. The various types of functional information obtained from advanced imaging modalities are listed below.

Table 7 Functional imaging techniques.

Imaging technique	Functional information
Perfusion CT	Provides information on the perfusion characteristics such as blood flow, blood volume, etc. Can guide the biopsy to avoid necrosis and maximize the yield Response evaluation after therapy
Dual energy CT	Characterize the tissue Iodine map to better evaluate tissue enhancement Can be used to evaluate lung perfusion
Apparent diffusion coefficient (ADC) in diffusion-weighted imaging (DWI)	Used to quantify the diffusion of water molecules Characterize tumors, detection of metastasis Response evaluation
Perfusion MRI	Evaluate pulmonary vasculature Quantification of perfusion parameters

Figs. 8A to C: Perfusion MRI showing a mass lesion in the right paracardiac region which was isointense on T1-weighted sequence (arrow) (A), hypointense on T2-weighted sequence (B). The periphery of the tumor is solid with increased perfusion whereas the center of the tumor is showing less perfusion (C). Regions of interest (ROIs) are placed over the periphery and center of the tumor which generated the perfusion curve and semiquantitative parameters (C).

CONCLUSION

Computed tomography and positron emission tomography-CT are the most commonly used imaging modalities used to evaluate and stage chest tumors. However, USG and MRI have their own advantages in selected clinical settings especially in children where radiation dose is a major concern.

Chapter Notes

Imaging Modalities in Chest Tumors: Role and TechniquesCHAPTER 1