W14. Medical Imaging of the Chest

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What do the ABCs of radiograph assessment stand for?

• Alignment

• Bone density

• Cartilage spaces

• Soft tissues

What can radiographs show about soft tissues?

Gross form such as muscle mass but not fine soft tissue detail

Why can soft tissues still be somewhat differentiated on radiographs?

There is enough variation in shades of gray to distinguish skin fat and muscle

What kinds of changes do radiographs usually show?

Significant changes

What is Computed Tomography (CT)?

An x-ray technology that creates very high-resolution thin slices of axial anatomy

How can CT images be displayed?

In different planes or reconstructions

What is the difference between a radiograph and a tomograph?

• A radiograph sees the whole body region (BOOK)

• A tomograph shows individual slices (INDIVIDUAL PAGES)

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Ex in photo: fx deep in the tibial plateau is easier seen on a CT scan

How are radiodensities of tissues represented on CT?

By shades of gray

How does air appear on CT?

Air → Black

• black Nike air forces

How does fat appear on CT?

Fat → Gray-Black

• fat g(r)ays wearing black

How does water appear on CT?

Water → Gray

• dirty gray water

How does bone appear on CT?

Bone → White

• white bones

How is the CT image displayed?

As though the patient is supine and you are looking through the feet in axial view

What is windowing in CT?

• A contrast enhancement technique that lets you visualize groups of radiodensities one at a time

• Humans see 32 shades of gray, computers see thousands

• CT scan can differentiate a 1% change in radiodensity, but it cannot be put all in one image, so it uses “windows” to visualize groups of radiodensities one at a time

Why is windowing important in CT?

It gives CT high sensitivity by emphasizing different tissue density ranges

What are the common CT windows?

• Bone window and soft tissue window

• Will show the ranges of raiodensities based on chosen tissue window

What does window level allow?

Discrimination between tissues of similar densities

What can a soft tissue window show well?

Soft tissues such as brain gray and white matter

What can a bone window show well?

Cortical bone and cancellous bone

What are common clinical indications for CT?

• Life-threatening injury evaluation

• Complex & subtle fractures

• Loose bodies in a joint

• Degenerative changes

• Spinal stenosis

What are the major advantages of CT?

• Fast image acquisition time

• Images bone and soft tissue in one series

• Can view large body regions

• Best in emergencies

• Excellent bone definition

Why is CT often preferred over MRI for cortical bone?

• Better cortical bone definition

• Less time-consuming ⏰

• Less expensive 💰

• Less claustrophobia

• Metal implants are generally less of an issue

What is the major disadvantage of CT?

High radiation exposure

What is MRI?

A cross-sectional imaging technique using a magnetic field and radiofrequency signals to generate images from hydrogen nuclei in water molecules

How are coronal MRI images viewed?

From the front facing the patient

How are sagittal MRI images viewed?

From one side of the body to the other

How are axial MRI images viewed?

From the feet toward the head like CT

What are the three basic components of MRI?

1. Scanner

2. Operator console

3. Computer

• The SOCk

How does MRI work in basic terms?

• Scanner creates a small magnetic field

• Hydrogen protons in water molecules in tissues align with a magnetic field

• Then a radiofrequency pulse disrupts them, then released energy is converted into an image

Why do different soft tissues look different on MRI?

Different tissues have different water content and release absorbed energy at different rates

What determines the construct of an MRI image?

The sequence used

What are the two common MRI sequences?

T1-weighted and T2-weighted

What is a basic tenet of MRI?

• Define anatomy (T1)

• Detect abnormal fluid (T2)

  • Hint: H2O → T2

  • White = edema in photo

How does pathology often appear on MRI?

Pathology commonly involves edema or inflammation

What does high signal intensity mean on MRI?

Brighter image

What does low signal intensity mean on MRI?

Darker image

How does water or edema usually appear on T2-weighted MRI?

Bright

What are common clinical indications for MRI?

• Soft tissue injuries

• Bone marrow changes

• Intervertebral disc pathology

What is a limitation or disadvantages or contrindications of MRI for bone?

• Cortical bone is not well seen because of low signal intensity

• Longer time

• Claustrophobia

• High cost

• Metal implant contraindications

What can ultrasound diagnose in vascular medicine?

Arterial and venous disease throughout the body

How can ultrasound be used in procedures?

To guide nerve blocks

How is ultrasound used in ophthalmology (eyes)?

To measure eye length and see inside lesions

How is ultrasound used in otolaryngology (ears, nose, throat)?

It is the preferred imaging modality for thyroid tumors and lesions

How is ultrasound used in newborns?

To assess the brain

How can ultrasound be used in pulmonary or thoracic care?

To visualize lungs in diagnosis of lung cancer

How is ultrasound used for the bladder?

To check retained fluid volume

How is ultrasound used in MSK imaging?

To image soft tissues such as tendons ligaments and nerves

What are major general features of ultrasound?

• Noninvasive

• No known ill effects

• Provides dynamic information

What is ultrasound imaging based on?

Tissue absorption of sound waves and reflection at tissue interfaces

What are the three components of diagnostic ultrasound?

1. Pulser

2. Transducer

3. Scan converter or monitor

What does the pulser (component 1/3) do in ultrasound?

Produces waves of electrical energy

What does the transducer (component 2/3) do in ultrasound?

Converts electrical pulses to sound waves and receives reflected waves back from tissues

Does diagnostic ultrasound increase tissue temperature?

No

Do the patient and imager have to remain static during ultrasound?

No

How are ultrasound images usually oriented?

In short axis and long axis relative to the structure examined

Why do tissues not have fixed ultrasound appearances like on CT or MRI?

Reflection intensity depends on the:

• Tissue interface

• Beam angle

• Tissue type

What does echogenic mean?

Tissues ability to return an echo or signal during ultrasound

What does hyperechoic (bright) mean?

• Tissues that reflect a lot of sound energy

• Bright

What does hypoechoic (dark) mean?

• Tissues that reflect little sound energy

• Dark

What does anechoic mean?

No reflection

What is one of the best uses of ultrasound?

Imaging soft tissue structures (tendons, ligaments, & nerves) in a dynamic functional state

What are disadvantages of ultrasound?

• Operator dependent

• Does not penetrate bone

• Cannot cross air interfaces

• Hard to image obese patients

Which modality is best for bone imaging?

Bone → CT

Which modality is best for soft tissue imaging?

Soft tissue → MRI

Which modality can rival MRI for soft tissue imaging in some cases?

2nd best for soft tissue → Ultrasound

What are the general advantages of a chest x-ray?

• Quick

• Cheap

• Noninvasise

• Useful for thoracic and nearby structures (thyroid, GI, bony thorax)

Why is the bony thorax important on chest radiograph?

Bones help identify:

• Soft tissue anatomy

• Lesion location

How is thoracic spine radiography different from chest x-ray?

• Thoracic spine → HIGH contrast & A-P

• Chest x-ray → LOW contrast & P-A

  • PA-PA’s chest

What chest x-ray view is usually preferred when possible?

PA

• double check this

Why is PA preferred over AP for chest x-ray?

AP magnifies the heart and lungs

What happens if the patient cannot stand for a chest x-ray?

The film is taken AP

How many anterior & posterior ribs are typically seen on an adequate chest x-ray?

• 6 anterior ribs

• 10 posterior ribs

Which ribs are usually easiest to count on chest x-ray?

Posterior ribs because they are furthest from the image receptor and can be traced back to the vertebrae → magnified the greatest

What happens to the anterior ribs on chest x-ray?

Anterior ribs become more radiolucent near the costal cartilages

What respiratory structures are commonly identified on chest imaging?

• Larynx

• Trachea

• Bronchi

• Lungs

What is the pleura?

A double-walled sac surrounding each lung

What is the parietal pleura?

The outer pleural layer lining the chest wall and diaphragm

What is the visceral pleura?

The inner pleural layer covering the lung surface and dipping into fissures

What is the pleural space?

The space between the parietal and visceral pleura

What is the cardiothoracic ratio?

A radiographic estimate of heart size

What is a normal adult cardiothoracic ratio?

The heart width should be less than half the chest width at the level of the diaphragm

What is the mediastinum?

The space between the lungs bounded by the sternum anteriorly and spine posteriorly

What major structures are in the mediastinum?

• Heart

• Great vessels

• Trachea

• Esophagus

All have similar radiodensity, besides air filled

What mainly forms the right mediastinal border?

• Superior vena cava

• Right atrium

What are the three major bumps on the left mediastinal border?

• Aortic arch or knob

• Left atrial appendage

• Left ventricle

What is the hilum?

The lung root of the mediostinum where bronchi, arteries, veins, and nerves enter and exit the lungs

What can hilar (hilum) enlargement suggest?

• Lymph node or tumor inflammation

• Pulmonary venous or arterial hypertension

What disease example in the notes can cause hilar enlargement from lymph node swelling?

Sarcoidosis

What is the diaphragm?

A thin curved sheet of muscle separating the thoracic and abdominal cavities

What is a costophrenic angle/sulcus?

The sharp recess where the diaphragm meets the chest wall

Why are the costophrenic angles/sulcus clinically important?

Pleural fluid collects there

What is a hemidiaphragm?

A term used to describe a half of s diaphragm because they often act seperately

• Split L + R

Where is the liver located relative to the hemidiaphragms?

Under the right hemidiaphragm

• R + Liver = River

Where is the stomach located relative to the hemidiaphragms?

Under the left hemidiaphragm

• Stomach took an L

At about what rib level should the diaphragm reach on a good inspiratory chest x-ray?

Around the 10th posterior rib

What can cause a flattened diaphragm?

• Any condition that increases lung volume

• Ex: Emphysema or large pleural effusions

What can cause an elevated diaphragm?

• Excessive peritoneal fluid from ascites, cirrhosis of the liver, or phrenic nerve injury

What is a hiatal hernia on chest imaging?

Part of the stomach herniates upward through the esophageal hiatus

What does free air below the hemidiaphragms on imaging mean?

Free air outside the bowel in the peritoneal cavity, usually suggesting bowel perforation

What gas is normal to see in the abdomen on imaging?

Gastric or bowel gas inside the GI tract