User:Yesh Doctor/sandbox

= M-Mode Ultrasound Imaging =

Introduction
M-mode (motion-mode) medical ultrasound (US) imaging displays successive A-mode ultrasound images as a single image by stitching these images together along the time axis. This creates an US “video” of sorts.

Typically, a B-mode ultrasound image is used first to properly position the ultrasound transducer. Many ultrasound systems allow for both the B-mode and M-mode images to be displayed simultaneously, with a dashed line across the B-mode image representing the axis of M-mode imaging (sometimes called the M-mode icepick).

M-mode US was initially developed as an examination and diagnostic tool for cardiac imaging by Edler and Hertz in 1953. M-mode is particularly useful for visualizing heart valve motion in real time. M-mode imaging is also used to measure abdominal muscle thickness during activity and diaphragm position. Due to its fast imaging speed and high frame rate, M-mode imaging can be critical in medical emergency applications where imaging time is limited; cases such as pneumothorax, cardiac tamponade, and hypertrophic cardiomyopathy.

Signal Generation
An A-mode (amplitude-mode) image is generated by a single transducer element firing a pressure wave envelope into the skin. The tissue-reflected pressure envelope is converted into a voltage through a piezoelectric element. This signal is displayed as the amplitude of voltage vs. time, generating an A-mode image. A M-mode image is generated by repeatedly taking the same A-mode image over a period of time. The M-mode image has two axes: a time axis and a depth axis. The time axis is defined by the time at which each A-mode image is taken, where the maximum time is equivalent to $$Nt_0 $$ with $$N$$ being the total number of A-mode images and $$t_0 $$ being the time to take a single A-mode image. Each A-mode image becomes a column (row of data along the time axis) of the resultant M-mode image, where the amplitude of the voltage signal is the resulting brightness.

Imaging Depth
The imaging depth of an M-mode image can be controlled by changing the frequency of the ultrasound wave used. Generally speaking, the imaging depth is inversely related to the frequency of the ultrasound wave. Higher frequency waves are attenuated more easily, resulting in a shorter imaging depth. The following relationship describes the depth-dependent attenuation of ultrasound waves:$$A_z = A_0\exp(-\mu_az)$$Where $$A_{z} $$ is the resultant amplitude after traveling a depth $$z $$, $$A_{0} $$ is the initial amplitude, and $$\mu_{a} $$ is the amplitude attenuation factor (typically in Np/cm). This can be converted into a signal loss:$$L=-20log(\frac{A_z}{A_0})$$Where a loss $$L $$ of 80 dB is typical. Interpreting this as frequency-dependent attenuation: $$d_p=\frac{L}{2af}$$Where $$d_p $$ is the maximum imaging depth, $$f $$ is the frequency, and $$a $$ is a tissue specific attenuation factor. Typical $$a $$ values include 0.63 dB/cm*MHz for fat and 20 dB/cm*MHz for bone. From the above formula, it is clear that for the same loss and $$a $$, a higher frequency results in a smaller imaging depth. The type of tissue being imaged also plays a role in the maximum imaging depth. Bone is notoriously difficult to image due to an $$a $$ values around 40 dB/cm*MHz

Frame Rate
In order for successive ultrasound pulses (A-mode images) to be made, each successive pulse may only be made after all echoes from a previous pulse have died out. The time for this to occur is represented by $$T_R\geq \frac{2d_p}{c}$$where $$T_r $$ is the time between repeated images, $$d_p $$ is the maximum depth of wave penetration, and $$c$$ is the average speed of sound in the imaged material. A typical value used for $$c$$ is 1540 m/s. The upper bound on the frame rate at which successive images can be taken is given by $$f_r = \frac{1}{T_r} $$where $$f_r$$ is the frame rate.

Axial and Temporal Resolution
Along each column (A-mode image), the resolution of an M-mode ultrasound image is solely dictated by the axial resolution of the ultrasound system, as each image comprising the M-mode image is an A-mode image. The ultrasound transducer emits a wave envelope into the tissue, and the theoretical maximum resolution of the system is half of the spatial pulse length.

M-mode imaging has great temporal resolution due to imaging along a single axis. For example, consider taking an M-mode image of the mitral valve which would require a maximum imaging depth of ~6cm. This translates to a time per image of ~$$7.8*10^{-5} $$s, or a frame rate of ~ 12.8 kHz. Atrial fibrillation is likely the fastest phenomenon that will need to be recorded via M-mode imaging, and that occurs (at maximum) at ~400 bpm or ~7 Hz. M-mode imaging can provide excellent temporal resolution due to its high frame rate.

Pneumothorax
Pneumothorax, or a collapsed lung, is a phenomenon in which air enters the pocket between the pleural lining of the lung and the chest wall, compressing the lung. Pneumothorax is reliably diagnosable with X-ray projection radiography, however M-mode ultrasound can be a substitute. To diagnose pneumothorax, the ultrasound transducer probe is placed in either the second, third, or fourth intercostal space in the transverse position. Typically, the sonographer (ultrasound operator) will take a B-mode image to line up the A-mode images that will be used to create the M-mode image (ice pick). The A-mode image will cut across the chest wall, through the pleural line and into the lung.

Pneumothorax is typically indicated by a lack of movement in the lung due to a buildup of pressure in the chest cavity. The term "seashore sign" is used to describe a normal state, wherein the lung image appears fuzzy due to its motion up to the pleural line (a sandy beach), whereas the chest cavity appears striated and stagnant (the sea). The term "stratosphere sign" or "barcode sign" is typically used to described a diseased state, wherein both sides of the pleural line appear as clear striations. This is due to lung motion being restricted, leading to consistent imaging across time in the M-mode ultrasound creating a barcode like pattern on the resultant M-mode image. While a M-mode image is not a sure fire diagnostic technique for pneumothorax (as a similar image will be seen in the case of pleural effusion), the fast nature of M-mode imaging makes it useful to gather initial results.

Cardiac Tamponade
Cardiac tamponade is an exacerbation of a pericardial effusion to a stage of ventricular collapse. The pressure developed from fluid pooling in the pericardial cavity can cause excessive pressure on both the left and right ventricles during diastole, resulting in inadequate blood ejection during systole. A key hallmark of cardiac tamponade is right ventricular collapse while the mitral valve is open. To diagnose cardiac tamponade, the patient is instructed to lay on their back and an ultrasound transducer probe is placed such that a parasentral long axis view (PLAX) of the heart can be taken. This takes a side-view of the heart, where the A-mode ice pick cuts across the pericardium, right ventricle wall, inter-ventricular septum, mitral valve, and the left ventricle.

Taking M-mode images using a PLAX view shows how the right ventricle wall moves in relation to the mitral valve. Typically there should be little to no movement of the right ventricle. However, during cardiac tamponade the right ventricle wall tends to move with the mitral valve. This is because mitral valve opening is commensurate with diastole (right and left ventricular filling), and the pressure due to the pericardial effusion is compressing the right ventricle in while it is filling. Movement of the right ventricle free wall with the mitral valve is a strong indication of cardiac tamponade.

Left Ventricle Systolic Function
Mitral valve E-Point Septal Separation (EPSS) is a common diagnostic technique used to assess left ventricle health. During systole, the mitral valve creates two characteristic waves in its position; the E-wave (or early filling wave), and A-wave (atrial kick). These waves are predicated upon a pressure difference between the left atrium and ventricle. Poor left ventricular function leads to incomplete ejection of blood in the left ventricle, decreasing the pressure gradient between the left ventricle and atrium, increasing the distance between the mitral valve and inter-ventricular wall. EPSS measures the distance between the peak E-point and the inter-ventricular septum.

To take an EPSS measurement, the patient is placed in the same position as they would be for PLAX imaging (cardiac tamponade imaging). The physician can read the EPSS off the M-mode image as it provides a depth scale. An EPSS of <6mm is normal, 6-12 is a moderately diseased state, and >12mm is a severely diseased state.

Diaphragmatic Paralysis
A paralyzed diaphragm is due to weak phrenic nerve signaling to the diaphragm resulting in in the stagnation of diaphragm movement; symptoms include shortness of breath, lack of control over voluntary breathing, and hypoxia. Patients with diaphragmatic paralysis tend to require ventilators.

M-mode imaging has been used to diagnose both adults and children with diaphragmatic paralysis. Patients lay supine, while the ultrasound transducer is placed in the intercostal space along the anterior axillary line, with the diaphragm being ~90 degrees to the transducer head. Patients are informed to take a normal breath followed by a sniff. A normal diaphragm will show motion on the M-mode ultrasound whereas movement will be absent in the case of a paralyzed diaphragm.

Limitations
The primary limitation of M-mode imaging is that all images are effectively one dimensional (line images). M-mode gives no information about the larger biological context; it simply provides information along a single axis. Additionally, measured quantities from M-mode images are not perfectly representative of true distances, as an average speed of sound in the body is used to determine distance. Current ultrasound technology (hardware and software) has advanced to the point where B-mode imaging with doppler techniques can provide more accurate results.