Healthy volunteers will be asked to complete a single imaging session that will include acquiring simultaneous 4D ultrasound and 4D MRI scan.

A substudy including patients undergoing radiotherapy for malignancies of the abdomen and pelvis. They will also be asked to complete a single imaging session that will include acquiring simultaneous 4D ultrasound and 4D MRI scan.

Patients receiving radiotherapy for malignancies of the abdomen and pelvis will be asked to complete two research visits lasting approximately one hour. Each visit will include a 30 minute combined 4D ultrasound and 4D MRI scan.

This 4D planar US transducer is being developed and validated to provide a novel image-guidance technique to directly track tumor motion during radiation therapy that is coupled to a pre-treatment calibration training image set consisting of a simultaneous 4D ultrasound and 4D magnetic resonance imaging (MRI) acquisition.

To conduct non-contrast agent MR and ultrasound scans in order to evaluate and demonstrate, on a continuing basis, the technical and clinical feasibility of new or modified features (the "Prototype") of a combined MRI and ultrasound system, and software. Prototype MRI and ultrasound systems are defined as components of the MRI system (software) and ultrasound system (hardware and software) that have been modified in some form.

The goal of this trial is to assess the feasibility of implementing the system into clinical practice and to compare the dosimetric advantages of its use. In a retrospective analysis, the patients' standard treatment plan with international target volume (ITV) expansion will be directly compared with a second treatment plan using a smaller respiratory-gated margin (i.e., gross tumor volume (GTV)-only expanded for planning target volume (PTV) without ITV expansion). Normal tissue constraints will be specified and follow standard clinical constraints. The percent of liver volume receiving greater than 18 Gy will be used as a quantitative metric to assess the dosimetric impact of our platform. In addition, overall treatment time with the relative "beam on time" and duty cycle will also be compared. Additionally, the ultrasound acquisition during treatment will be used to assess intra- and interfraction changes in breathing patterns as the patients progress through treatment.

The accuracy of our image-guidance technique will be done by showing that several salient features identified in the US scan can be used to predict/model target motion identified in the MR scan. In this context the target will be a large easily identifiable structure in the test subject (e.g. cyst, vessel). MRI and ultrasound images will be acquired simultaneously such that the motion of tissues or organs in the body can be effectively tracked as a function of respiratory state in the subject. In this manner, the ultrasound system will allow real-time tracking of salient features in the patient that are indicative of the respiratory state.