NMR field probes are increasingly used to measure the spatiotemporal magnetic field evolution during MR procedures^1,2. The sensor heads are typically formed by enclosing a 1H or 19F NMR-active liquid in a capillary surrounded by a solenoid coil for signal excitation and detection. The sensitive volume is shaped by the solenoid's spatial transmit and receive characteristics. One key downside of this approach is that the resulting effective droplet is only softly defined, resulting in a suboptimal trade-off between signal yield and k-space range that can be covered without probe de-phasing. What is more, the inhomogeneous transmit field amplitude causes a flip angle gradient throughout the excitation volume, which reduces the SNR. For optimal performance the sensitive volume should be spherical, sharply delineated, and fully excited with uniform flip angle. In the present work this is achieved by the transition to physical droplets suspended in a gelled medium combined with adiabatic excitation. By using BIR-4 ^3,4 adiabatic plane rotation pulses the excitation flip angle can be exactly controlled.19F NMR field probes with spherical sample were built according to the schematic shown in Fig. 1. The sample is a hexafluorobenzene droplet enclosed between D₂O plugs in a glass capillary of either 1.3 mm or 0.8 mm inner diameter. To shape and immobilize the droplets the plugs were gelled by adding 1% Imagel 69157 (Gelita AG, Germany). Their magnetic susceptibility was matched to the droplets using MnCl₂∙4H₂O. The sample liquids were doped with Cr(tmhd)₃ (48.7 mM) to reduce T₁. To compare performance, conventional virtual droplet probes² were built using the same sample liquid and dimensions. Probe excitation was performed alternatively with a 5 µs block pulse, a 48 µs adiabatic half-passage (AHP, 90°) and a 48 µs BIR-4 (90°) adiabatic plane rotation pulse of varying power. To include tolerance to small frequency offsets, hyperbolic secant (HSn) adiabatic pulses were used ⁵. Measurements were performed at 3T in a Philips Achieva MRI scanner and the 3D gradient echo images of the droplet were acquired at 4.7 T using a Bruker PharmaScan animal MRI scanner.Figure 1 (right) illustrates the successful formation of highly spherical probe droplets as confirmed by high-resolution MRI. Due to the spherical shape of the sample the probe signals exhibit the same drop-off in all gradient directions (Fig. 2 left) which is not the case for the virtual droplet probes (Fig. 2 right). With the physical droplet the signal decay under gradients along the capillary is less steep initially and steeper towards zero, resulting in substantially greater k-space range for any given signal threshold. The impact of pulse choice is summarized in Fig. 3. We varied the B₁ amplitude of the pulse and measured the initial absolute value of the FID. This results in a curve proportional to the sine of the flip angle achieved at the corresponding B₁ amplitude, meaning the peak value represents 90° flip angle. Adiabatic excitation robustly yields maximum signal from sufficient B₁ amplitude upwards. Conventional block pulse excitation naturally suffers from partial under- and over-flipping due to non-uniform B₁, making the flip angle sensitive to the B₁ amplitude (Fig.3). Importantly, the proposed physical droplet boundaries prevent expansion of the excited volume and thus deterioration of k-space range with increasing adiabaticity (Fig. 4). In the virtual droplet probe, on the other hand, enhancing B₁ enlarges the excited sample volume, which results in reduced k-space range. With BIR-4 pulses any specified flip angle can be reliably induced as demonstrated in Fig. 5.The trade-off between signal yield and k-space range of NMR field probes can be optimized by forming spherical fluorocarbon samples in a gelled polar medium. Sharp droplet delineation permits the use of adiabatic pulses for uniform excitation irrespective of gradient exposure. Perfect adjustment of the flip angle permits experiments with smaller flip angles and faster re-excitation. Improved field probe performance will be instrumental in the ongoing development of field cameras for system diagnostics, image reconstruction, and real-time field control.