A-mode (Amplitude-mode) ultrasound is a technique used to assess organ dimensions and determine the depth of an organ. While A-mode technology was previously employed in midline echoencephalography for rapid screening of intracranial mass lesions and ophthalmologic scanning, it is now considered obsolete in medical imaging. Nonetheless, the A-mode scan has found applications in early pregnancy assessment (specifically the detection of fetal heartbeats), cephalometry, and placental localization.
When the ultrasound beam encounters an anatomic boundary, the received sound impulse is processed to appear as a vertical reflection of a point. On the display, it looks like spikes of different heights (the amplitude). The intensity of the returning impulse determined the height of the vertical reflection and the time it took for the impulse to make the round trip would determine the space between verticals. The distance between these spikes can be measured accurately by dividing the speed of sound in tissue (1540 m/sec) by half the sound travel time.
During an echoencephalography scan, the first A-mode scan is acquired from the right side of the head and captured on film. Subsequently, the probe is positioned at the corresponding point on the left side, and a second exposure is captured on the same film, displaying inverted spikes. The A-mode ultrasound could be used to identify structures normally located in the midline of the brain such as the third ventricle and falx cerebri. The midline structures would be aligned in normal patients but show displacement in patients with mass lesion such as a subdural, epidural, or intracranial hemorrhage.

See also 2D Ultrasound, 3D Ultrasound, 4D Ultrasound, Ultrasound Biomicroscopy, A-scan, B-mode and the Infosheet about ultrasound modes.

Gynecologic ultrasound and obstetric ultrasound are two distinct applications of ultrasound imaging that serve different purposes in the field of women's health. While both involve the use of ultrasound technology to examine the pelvic region, they have different focuses and objectives.

Gynecologic [gynaecologic, Brit.] ultrasound primarily concentrates on the evaluation of the female reproductive organs, including the uterus, ovaries, fallopian tubes, and surrounding structures. It is commonly performed for various gynecological concerns, such as abnormal bleeding, pelvic pain, infertility investigations, and monitoring of reproductive disorders. It can identify signs of inflammation, the presence of free fluid, cysts, and tumors. This non-invasive technique aids in diagnosing and monitoring gynecological pathologies, facilitating early intervention and appropriate treatment. Typically, a transabdominal sonogram is performed with a full bladder to provide an initial assessment. However, if the pelvic ultrasound reveals any abnormalities or fails to provide a clear image of the organs, a more detailed evaluation can be achieved through a transvaginal sonography. This approach allows for improved visualization of the uterus and ovaries by placing the ultrasound probe inside the vagina.

Obstetric ultrasound, also known as prenatal, fetal or pregnancy ultrasound, is the branch of medical imaging that focuses on the use of ultrasound technology to assess the health and development of a fetus during pregnancy. Women with uncomplicated pregnancies commonly undergo an ultrasound examination between the 16th and 20th week of gestation. This routine assessment, performed with a real-time scanner, serves to determine accurate gestational age, monitor fetal size, and assess overall growth. The middle of the pregnancy trimester provides a crucial window for detecting many abnormalities of fetal anatomy. Advanced imaging techniques enable healthcare professionals to identify potential structural issues. Early detection of these abnormalities allows for timely intervention, counseling, and the implementation of appropriate management strategies.
See also Pregnancy Ultrasound, Pelvic Ultrasound, Hysterosalpingo Contrast Sonography and Vaginal Probe.

Interventional ultrasound, also known as ultrasonography, encompasses a range of invasive or surgical procedures guided by ultrasound imaging. While its widest application lies in intravascular ultrasound imaging for measuring atherosclerotic plaque, it has proven valuable in various medical fields.
In urology, ultrasound-guided interventions are employed for treatments like high intensity focused ultrasound (HIFU) in prostate conditions. The precise imaging provided by ultrasound aids in targeting the affected area and delivering therapeutic energy effectively.
In intraabdominal conditions, endoscopic ultrasound is frequently utilized. This technique combines ultrasound imaging with an endoscope to visualize and evaluate structures within the gastrointestinal tract, allowing for precise diagnoses and targeted interventions.
Ultrasound-guided procedures play a significant role in several medical specialties, including liver sonography, obstetric and gynecologic ultrasound, and thyroid ultrasound. These procedures involve interventions such as RF thermal ablation or biopsies, which are guided by real-time ultrasound imaging.
For instance, in liver sonography, ultrasound guidance is crucial for performing biopsies or RF thermal ablation, a technique used to treat liver tumors by delivering localized heat to destroy the abnormal tissue. The real-time imaging allows for precise needle placement and monitoring during the procedure.
In obstetric and gynecologic ultrasound, ultrasound-guided procedures, such as biopsies, can be performed to obtain tissue samples for diagnostic purposes. Additionally, ultrasound guidance is valuable during interventions like amniocentesis or fetal blood sampling, enabling accurate and safe procedures.
Thyroid ultrasound procedures often involve ultrasound-guided fine-needle aspiration biopsy (FNAB), which allows for the sampling of thyroid nodules for cytological examination. The ultrasound image helps guide the needle into the targeted area, ensuring accurate sampling and minimizing potential complications.
Overall, ultrasound-guided interventions provide minimally invasive and precise approaches to diagnosis and treatment. The real-time imaging capabilities of ultrasound contribute to enhanced accuracy, safety, and patient outcomes in procedures like biopsies, injections, and drainage.

See also Transurethral Sonography, Endocavitary Echography, and B-Mode Acquisition and Targeting.

Lateral resolution is the minimum separation of two interfaces aligned along a direction perpendicular (objects that are side by side) to the ultrasound beam. The lateral or angular resolution directly relates with the collimation of the beam emitted by the crystal. Lateral resolution is proportionally affected by the frequency, the higher the frequency the greater the lateral resolution.
Higher frequency transducers are used in fetal and pediatric echocardiography because the lateral resolution displays the smaller structures better. Lower frequencies are used for adults where structures are larger and the need for greater depth penetration is important.

The M-mode (Motion-mode) ultrasound is used for analyzing moving body parts (also called time-motion or TM-mode) commonly in cardiac and fetal cardiac imaging. The application of B-mode and a strip chart recorder allows visualization of the structures as a function of depth and time. The M-mode ultrasound transducer beam is stationary while the echoes from a moving reflector are received at varying times.
A single beam in an ultrasound scan is used to produce the one-dimensional M-mode picture, where movement of a structure such as a heart valve can be depicted in a wave-like manner. The high sampling frequency (up to 1000 pulses per second) is useful in assessing rates and motion, particularly in cardiac structures such as the various valves and the chamber walls.