1)   Permanent magnets

Characteristics:

• Made of ceramics
• Easy to magnetized
• Lightweight
• Not expensive
• Produces 0.3 Tesla (Magnetic Field Strength)
• Disadvantage: Difficulty in producing uniform magnetic field strength

2)   Resistive

Characteristics:

• Made of multiple coil system
• Requires rigid framework
• Needs huge power consumption
• Needs water cooling system
• Produces 0.3 Tesla (MFS)
• Disadvantage: High heat production

3)   Super conducting

Characteristics:

• Coil system made of super conducting metal alloy
• Possess no resistance
• No water cooling system
• No need for large high current PS – lesser power consumption
• Produces 2 Tesla (MFS)
• Housed in DEWAR – highly insulated bottle

2 Chambers of DEWAR

v  Outer chamber – contains liquid Nitrogen

v  Inner chamber – contains liquid Helium

The following list below together with their overview of benefits are shown below:

1. Cerebral Angiography or Vertebral Angiogram or Carotid Angiogram: In this radiological diagnostic method a catheter (a long narrow flexible tube) is inserted into the patient’s artery (usually from his/her leg) with the help of a needle and guided through the main vessels of the abdomen and chest until it is placed in the arteries of the neck. The whole procedure is monitored with the help of a fluoroscope. X-ray pictures are taken after injecting a contrast dye in the neck with the help of the catheter.
2. Carotid Duplex or Carotid Ultrasound: In this noninvasive test, ultrasound waves are used to detect plaque, blood clots or other blood flow problems in the carotid arteries. The images of carotid arteries and pulse wave form are carefully observed on a monitor.
3. Computed Tomography (CT or CAT scan): This method is used to diagnose and detect hemorrhagic strokes, since blood, bone and brain tissue can be easily distinguished due to their difference in densities. It is important to note that damage from ischemic stroke cannot be detected by this scanning method.
4. Doppler Ultrasound: In this noninvasive test high frequency sound waves are directed with the help of a transducer on to the artery or vein in question and detected on the Doppler.
5. Electroencephalogram (EEG): In this method, small metal discs (electrodes) are placed on the patient’s scalp to detect electrical impulses, which are printed out as brain waves.
6. Magnetic Resonance Imaging (MRI): In this method of diagnosis, three dimensional images of body structures are produced using magnetic fields and computer technology. The clear pictures of nerve tissue, brain stem and posterior brain can help determine signs of prior strokes if any.
7. Magnetic Resonance Angiogram (MRA): The MRA shows actual blood vessels in the neck and brain and helps in detecting blockage and aneurysms.

Source: http://www.omnimedicalsearch.com/conditions-diseases/cerebrovascular-disease-tests-diagnosis.html

There is a new innovation in the world of Magnetic Resonance Imaging. It is specially designed for breast scans and has made the entire examination more enduring and comforting due to its compact, open design that has a diameter of only 70 centimeters, while the magnet bore is 10 centimeters wider as compared to most conventional systems.

This new innovation is a product of Siemens and was named Magnetom Espree-Pink. This 1.5-Tesla system is the shortest system available on the market today. With this breakthrough, patients can be positioned either feet-first or head-first and the scanner can also offer sufficient space for adipose patients.

Furthermore, the flexible “Sentinelle Vanguard for Siemens” coil solution makes both imaging and biopsies possible by only using a coil. Also, the coil can be optimally adapted to the different breast sizes of various patients due to its variable coil geometry (VCG). Excellent image quality is attained through an improved signal-to-noise ratio and with the help of eight RF channels.

Source: EH Online

Respiratory Gating

- the simplest method of eliminating respiratory motion artifact is to suspend respiration entirely though breath hold
- respiratory gating and respiratory compensation were utilized because not all patients can hold their breath long enough to acquire adequate images

Respiratory Compensation

- also known as exorcist, ROPE and COPE
- performed through reordered phase encoding
- throughout the respiratory cycle, anterior chest wall motion is monitored with pressure transducer that surrounds patient’s chest
- phase encoding steps are then reordered to decrease the intensity of respiratory motion artifacts and changes location of the motion artifacts within the data set
- has potential advantage over respiratory gating because acquisition tiem is not increased
- signal averaging results involved results in marked resolution loss and obscures fine detail
- a complicated real-time technique

Respiratory Gating

- MRI with respiratory gating is a simple and practical approach to reduce respiratory motion effects
- Data are collecting during continuous breathing but are used for image reconstruction only if they are collected within a reference range
- Before, reference for respiratory motion was obtained by placing a belt containing a displacement transducer around the upper abdomen
- Recently, navigation spin echo has been used to monitor diaphragmatic motion
- Acceptance or rejection of data can be made in real-time or retrospectively
- Disadvantage is increased imaging time

Cardiac Gating

- Used to reduce cardiac motion and can be monitored by measuring the ECG signal via placement of MRI-compatible electrodes on either the patient’s chest or back
- It is thought that placement of the electrodes dorsally may reduce artifact caused by lead motion
- If leads crossed out each other or looped, it can cause induction of undesired currents and the possibility of surface burns
- R-R interval is measured and image acquisition is triggered by the R-wave
- Alternative method is peripheral gating or plethysmography, where gating is triggered by the peripheral pulse via a small probe placed at the fingertip

Coils

- Standard gradient body coil and the phased array surface coil are the two coils most commonly used in thoracic imaging
- Phased array coil provide good central and peripheral imaging, maintain field homogeneity and improve SNR above that of the standard body coil
- Smaller flexible surface coils, and sometimes dedicated shoulder coils, are also used for imaging the superior sulcus and brachial plexus

Contrast Agents

- Intravenously administered gadolinium chelates is the most commonly use CM for thoracic MRI
- CM used in MRI are all paramagnetic agents that increase signal and have relaxivity rates
- Paramagnetic agents are administered prior to T1-wieghted image acquisition
- The one exception to this in the thoracic are double dose dynamic gadolinium-enhanced sequences
- Prior to administration of CM, patient’s medical history, specifically drug allergies must be evaluated

Specific uses

1. Aorta and Great Vessels
- MRI has become an important method of assessing dissection, aneurysms, pseudoaneurysms and congenital anomalies
- T1-weighted spin echo imaging with intravoxel dephasing is the most useful sequence for imaging the thoracic aorta and great vessels
- A newer method for MRI of the aorta and great vessels is dynamics double dose gadolinium-enhanced 3D imaging
- Injection is carefully timed to obtain images during peak bolus as determined by the initial test dose

2. Cardiac
- Should be performed in at least two planes
- 3D gradient-recalled echo imaging, a rapid technique, can also be used to image the heart
- Cardiac MRI is also performed to assess congenital disease
- Newer cine-2D GRE sequences can demonstrate flow patterns suggestive of valvular stenosis and regurgitations

• Within the thorax, the right wall of the trachea is in contact with the mediastinal pleural reflections of the right upper lobe, creating the right paratracheal stripe seen on posteroanterior chest radiographs.
• Potential space, also known as the retrotracheal recess, exists between the posterior right half of the trachea and the posteriorly positioned right lateral wall of the esophagus
• Retrotracheal recess is frequently occupied by the lung
• Fair amount of normal variability in this space can create interfaces that stimulate pathology in conventional roentgenography

Anterior Junction Line

• Corresponds to the area where the two lungs come into close approximation anteriorly
• The pulmonary artery and the ascending aorta are the posterior boundary of this line
• The anterior boundary is the anterior chest wall

Posterior Junction Line

• The region where two lungs lie close to each other posterior to the heart and trachea
• The right lung invaginates behind the right hilar structures and heart to contact the pleura overlying the azygos vein and the esophagus
• Paraspinal area are contagious with the posterior junction
• Contains intercostal vessels and small lymph nodes

Diaphragm

• Musculotendinous structure
• Separates the thorax from the abdomen
• Chief muscle of respiration
• The anterior portion is attached to the posterior surfaces of the six lowermost coastal cartilages and the sternum
• The pericardium is attached to the upper surface of the central tendon of the diaphragm
• Superior diaphragmatic (cardiophrenic angle) lymph nodes are located extrapleurally

3 major passages in the diaphragm

1. for the inferior vena cava
2. for the esophagus and vagus nerves
3. for the aorta and thoracic duct

• these three openings are not visible in transaxial images

Diaphragm is very hard to image because of the following:

1. large-dome portion is not discretely visible in the CT
2. cardiac motion interferes with distinct imaging of the horizontal portion of the diaphragm

• the most anterior portion has a variable appearance on the CT
• the anterior portion often appears as a relatively smooth slightly undulating soft tissue arc concave posteriorly and continuous across the midline
• the posterior and caudal portion, especially the crural part, is generally well-defined on the CT
• the most cephalad section is at the level of the esophageal hiatus

Normal Variants

Situs Inversus Totalis

• situs is a reference position of the thoracic and the abdominal viscera within the body
• situs solitus, the most common type of situs, indicates a trilobed lung, right atrium and liver on the right side and a bilobed lung, stomach and spleen on the left side
• Situs inversus refers to the mirror image of the thoracic and the abdominal viscera
• 20% of the patients with situs inversus have Kartagener syndrome, subset of the ciliary dyskinesia syndrome

Mediastinal Vascular Anomalies

• Includes anomalies of the intrathoracic vessels, including the superior vena cava, the aorta and the pulmonary arteries and veins
• In some patients, these anomalies may have simulated a mediastinal mass on plain chest radiographs
• CT can serve as a relatively noninvasive procedure for definitive identification of these anomalies or variants
• Occasionally, anomalies of isolated partial pulmonary venous drainage are detected on CT
• Other anomalies of pulmonary venous drainage include the right inferior pulmonary vein cephalad into the azygos vein
• Rarely, other forms of anomalous venous return can be recognized on CT or MRI, particularly with bright blood 2D GRE sequences used for the latter
• Anomalies of the inferior vena cave can be associated with dilatation of the azygos and/or hemiazygos veins
• Careful examination of the serial CT scan should enable correct diagnosis from the characteristics of findings

Arterial Anomalies

Can be generally categorized into 3 groups:

1. left aortic arch
2. right aortic arch
3. double aortic arch

• most common congenital anomaly of the aorta is an aberrant right subclavian artery originating from an otherwise normal left aortic arch
• the anomalous right subclavian artery is seen on CT arising as the last branch from the distal portion of the aortic arch
• most frequent anomaly of the right aortic arch is with an aberrant left subclavian artery
• right aortic ach with mirror image branching has a high incidence of associated congenital heart disease
• double aortic arch is characterized by 2 arches arising from a single ascending aorta
• typically, the right arch is larger and situated slightly more cephalad than the left arch

Isolated Left Vertebral Artery

• rarely, the left vertebral artery may arise separately from the aortic arch and an additional great vessel will be seen at this level

Vascular-Aortic Arch

• Pseudocoarctation, or kinking of the aortic arch, is a rare anomaly that may be mistaken for true coarctation of the mediastinal mass on conventional chest radiography
• Because proximal descending aorta is located anteriorly, on CT lung can be seen interposed between the descending aorta and the spine