The purpose of this study is to determine the feasibility and efficacy of detecting kidney transplant rejection using contrast enhance ultrasonography with the contrast agent Sonazoid.

This study will be divided into two stages. In the first stage the investigators will assess the transplant kidney with ultrasound after contrast injection (Sonazoid) to determine if contrast in the kidney is detectable by ultrasound and to determine if the amount of perfusion can be quantified directly or qualitatively. The investigators aim to have one to two subjects in this stage. If perfusion of the kidney can be assessed, then the investigators will move to stage two of the study.

In stage 2, the investigators will look to determine the ability of the contrast enhanced ultrasonography method to detect renal transplant rejection using Sonazoid. For this stage the investigators will recruit subjects with biopsy confirmed renal transplant rejection. Once subjects are consented, they will undergo contrast enhanced ultrasonography with Sonazoid and have images taken. These images will be compared to images of renal transplant patients that have displayed no rejection or clinical issues. These clinically normal subjects will have previously consented to a medical record review so that the investigators may use their images.

Inclusion Criteria:

• Adults (ages 18-75) who have undergone renal transplant at the Brigham and Women's Hospital. [For stage 1 or stage 2]
• Experiencing acute and chronic renal allograft rejection - defined by by examining the histological sections of renal transplant biopsy for a renal pathologist. This is the most accurate way to evaluate the presence of absence of acute and chronic renal transplant biopsy. [For stage 2]
• Adults (ages 18-75) who have undergone renal transplant at the Brigham and Women's Hospital with normal serum creatinine values (~1 mg/dl) [For stage 2 medical record review only]

Exclusion Criteria:

• Hemodynamic instability (e.g., blood pressure < 90)
• Atrial fibrillation with rapid ventricular response
• Arrhythmia
• Poor acoustic windows
• Inability to provide informed consent
• Known right to left or bidirectional cardiac shunts
• Any contraindication such as the history of allergic reactions
• Pregnant or nursing women